Testing Keisler’s Perfect Fit Kit

As modern engine swaps into classic musclecars become more and more popular, so does swapping in newer transmissions to match. Going from a tired factory automatic or three-speed stick to an up-to-date five- or six-speed overdrive transmission totally changes the character of any classic for the better. In the recent past, doing that kind of a swap could be an exercise in frustration, though; matching block to bellhousing, tunnel to trans, and crossmember to frame and exhaust took patience and more than a little fabrication skill. Fortunately, that’s no longer the case, and to prove it we installed a brand-new TKO behind the fresh GM LS crate motor in our Project Swinger ’71 Nova using the Keisler Engineering Perfect Fit Kit, a ‘transmission swap in a box’ that makes it almost easy.

The Nova in question is a non-console car that has been completely stripped down, ready for the installation of a modern drivetrain. The powerplant selected for this project is a GM Performance Parts LS376/480 crate engine. This 480 horse Gen IV small block is going to provide all the power we want for this street/pro touring Nova, but to properly harness it we need an appropriately modern transmission to match.

To that end, we knew we wanted to back our LS with a Tremec Transmissions TKO-600. Rated at 600 horsepower and 600 foot pounds of torque capacity, this gearbox would be capable of living happily with the power of the LS engine even if we decide to make some performance upgrades later. Thanks to it’s overdrive gear, we can also get the off-the-line performance we want with a low (numerically high) final drive ratio while keeping good fuel economy on the freeway.

But while the TKO is a great transmission, it was not designed to be bolted under the floor of an older musclecar like our Nova. Keisler Engineering realized this, and produced the Perfect Fit Kit, which changes the process from a one-off custom swap job into something easily accomplished by any home mechanic. When we say that this kit is everything you need to make the change, we mean everything. Check out what’s included…

Keisler Engineering Perfect Fit Kit Contents:

  • Transmission – Tremec TKO 600
  • Bell Housing – Keisler Tremec/LS-specific
  • Cross Member – Keisler Boxed Cross Member, Nova/Tremec TKO-specific
  • Shift Kit – Keisler-modified Tremec Shift Kit
  • Shifter – Keisler Ball and Stick
  • Flywheel – Keisler LS Flywheel
  • Clutch Kit – Hydraulic Clutch with Keisler Pedals and Pads and Wilwood Master Cylinder with Remote Reservoir
  • Driveshaft – Custom length Keisler Aluminum
  • Misc. Parts – Keisler Transmission Tunnel Template and Sheet Medal Cover Plate

A simple list doesn’t really do justice to everything that’s included, though, so let’s take a look at it all, step by step. Keisler offers Perfect Fit Kits for a lot of different early model GM, Ford, and Mopar applications, and with a range of transmissions from our TKO-600 to 4-speed automatics.

Transmission & Shift Kit

First of all, Keisler does more than just send you a random rebuilt junkyard trans – every transmission is sourced directly from Tremec, and goes through a few modifications before it’s shipped. “Each transmission is completely disassembled and checked for proper fit,” says Shafi Keisler, owner of Keisler Engineering. “If something isn’t fitting just right, we machine it so everything is exact,” he added. According to Keisler, the main aluminum cases are machined to help fit the larger Tremec transmission into the factory tunnel with as little modification to the sheet metal as possible. Also keeping the fabrication down is a uniquely designed shifter developed in-house that moves it to right under the factory tunnel hole to give you the shortest throws and the best overall fit.

You also get a back-up light wiring harness, neutral safety start switch and your choice of provisions for a mechanical or electronic speedometer cable depending on your preference.

Bellhousing, Clutch & Flywheel

Keisler includes a new bellhousing with the kit as well. This can be ordered with the exact bolt pattern you need for your engine – obviously, we selected one to match up to our LS block. Keisler also provides an appropriate flywheel for your chosen application, and upgrades to 4140 forged or billet flywheels are also an option.

The clutch included in our kit was a Pro Touring-style dual friction unit. “This design is a great overall clutch if you are running autocross or doing some street/strip work,” says Keisler. It uses an organic/copper friction material that is metal-backed for strength. A sprung hub works to dampen clutch vibration for smoother engagement, while the entire pressure plate is computer balanced. The clutch assembly even includes a sealed hydraulic release bearing. All of this works to give you a very strong but street-friendly clutch.

Clutch Actuator Kit

Instead of using a traditional cable or rod-operated clutch fork to handle disengaging the engine from the transmission, Keisler uses an OE-style hydraulically-actuated throwout bearing. “We chose to use a hydraulic clutch because it provided the most clearance inside the transmission tunnel,” explained Keisler. Instead of brackets, forks, pins, cables, and quadrants to deal with, there’s just a master cylinder mounted to the firewall and a single braided stainless line running to the transmission.

Keisler includes everything you need to retrofit the hydraulic clutch into your classic muscle car. A Wilwood master cylinder with a remote reservoir is supplied, and we were impressed to see these top-quality components included in the kit instead of some cheap offshore no-name parts. To adapt the master cylinder to your particular application, Keisler supplies a bracket that mounts the cylinder to the firewall in the stock location.

One nice thing about using a hydraulic clutch is the freedom you get in positioning the engine and transmission in the car. Unlike a cable-operated clutch, you are not bound by the length of the cable itself. With the hydraulic line, as long as both ends are linked to each end of the system, it will work the way it is designed to.

Shifter & Pedals

Inside, Keisler provides both brake and clutch pedals and a shifter that are designed to require little to no modification to the car to install. The shifter is a custom short throw unit made specifically for use with the TKO transmission. It includes the shift ball and all the hardware to bolt it together. Combined with the shift kit installed on the transmission, you end up with a shifter position that mirrors the stock transmissions that came with these cars, and as little modification as possible required to the transmission tunnel.

The pedal kit includes more than just the pedals themselves. You also get everything needed to mount them including an adjustable pushrod to link the clutch pedal to the master cylinder. Keisler did a nice job at choosing a pedal design that will blend right in with the look of any classic muscle machine.

Transmission Tunnel Template & Sheet Medal

As much as we might wish it would, the larger Tremec transmission will not just slide right in the tunnel of most classic cars. The same goes for many different late model transmission choices such as the Tremec T56. Modifying the transmission tunnel may seem like a huge job, but Keisler makes it extremely simple. They include a template that uses existing marks on the vehicle as a reference for positioning, making it easy to mark where the necessary cuts need to be made. Keisler doesn’t leave you with a huge hole in your floor either. They include a pre-bent plate that fills in for the missing sheetmetal perfectly.

Crossmember & Drive Shaft

For securing the transmission to the car, Keisler came up with their own crossmember to make it a bolt-in job. The crossmember mimics the OE piece when it comes to bolting up to the frame, but it’s designed to be used with the TKO transmission. The boxed steel crossmember is crafted to leave plenty of room for a large three-inch exhaust, too. “We spent a lot of time designing this piece to make sure it worked the way we wanted it to,” says Keisler. “In addition to holding the weight of the transmission, it also keeps the correct driveline angle and geometry.”

The final major component is the driveshaft. Made from lightweight high strength steel, this shaft comes already welded up and computer balanced, with solid sealed U-joints and a Spicer slip yoke. It’s the one component in the kit you won’t find included in the box, though. “Due to the differences in rear end choices and other factors, we hold off on sending the driveshaft until you are able to install the kit and measure for it,” Keisler explained. “This allows us to insure the perfect fit.”

Everything You Need, Nothing You Don’t

The Keisler Perfect Fit Kit puts together literally everything necessary to connect your engine to your axle under one part number, and better yet, you are assured that all the components will work together correctly – a major plus for those of us who are not transmission experts. There’s really no easier way to put a late-model transmission into a classic Chevy, and to prove it, follow along as we do exactly that.


Installing this kit is nearly as easy as ordering it. We managed to complete the entire job in just under a day’s worth of work. This was mainly because the car was stripped down with no fenders and no engine or transmission to pull out of the way. Having a two-post lift didn’t hurt either, but that doesn’t mean you need a high end shop to install this kit. There is no reason why this couldn’t be done in the driveway with a good set of jackstands, a transmission jack adapter, and a portable MIG welder as long as you set aside a weekend to do it.

We began our install by bolting up the new clutch and flywheel to the back of our LS using the supplied hardware, then securing the transmission to the block once everything was lined up properly.

With Keisler’s bellhousing, there’s no trick to mating the TKO to our LS – just bolt and go.

Here, it’s easy to see how the hydraulic throwout bearing on the clutch makes for a much simpler setup than a traditional mechanical clutch fork and cable.

Of course, before we could slip the powertrain into the car, we still needed to open up the transmission tunnel to fit the new Tremec TKO-600. The included template shows you exactly where to make your cuts to give the new transmission all the room it needs. To fill the hole, Keisler provides new sheet metal that is already notched and bent to fit. Unfortunately, our car was a little hacked up from the previous owner, so we had more patching to do than will typically be required. Once the rear part of the transmission tunnel was looking more like it did from the factory, the cover plate was welded over the newly-cut section.

With the previously-chopped-up tunnel patched, we cut per the template and welded in the Keisler-supplied panel to accommodate the new TKO. Once it’s been primed and painted, new carpet will make it look like it came that way from the factory.

Since we needed a way to convert from the tower-style LS engine mounts to standard SBC mounts, we picked up a LS Engine Swap Kit from BRP Hot Rods. Like the Keisler kit, BRP’s mounts are designed to bolt right up to the stock holes in the subframe and engine block.

Included in the BPR Muscle Rod Kit:

• Two Bolt-In Frame Mounts
• Two Motor Brackets
• Energy Suspension Transmission Mount
• Transmission Crossmember
• Two Crossmember-to-Frame Brackets
• Optional Hedman Hedders

The BPR engine mounts allow our LS to bolt to the Nova just like any other small block Chevy, and they put the engine down deep between the framerails – good for both a low center of gravity and hood clearance as well.

The BRP Kit we selected also includes a pair of Hedman Hedders. Hedman offers seven different Hedder part numbers that cover popular GM musclecar LS swaps, ranging in size from 1 3/4” to 2” primaries. In our case, 1 7/8” ceramic-coated Husler Hedders with 3 inch collectors were chosen.

The Keisler transmission crossmember bolts to the existing holes in the frame, just like the factory piece. While having a lift makes the job easier, it’s still very possible to do this swap in a driveway with the car securely supported on a good set of jack stands.

With the transmission in place, we were able to measure for the driveshaft and send the information to Keisler so it could be custom-made to fit.

Back under the hood, we mounted the clutch reservoir and master cylinder to the firewall in the stock location and connected the hydraulic line.

Under the dash, we hung the new pedal assembly which bolts directly in place of the stock piece, then connected it to the clutch master cylinder. With the reservoir filled and a few pumps to bleed the system, our new clutch was operating smoothly.

Finally, we attached the shifter to the transmission with the two supplied bolts, and screwed down the shifter knob.

Overall, the Keisler Perfect Fit Kit provides an easy way to transplant a modern transmission into your classic musclecar, no matter your engine choice. In a single day we were able to complete the installation, compared to the days it would have taken to fabricate this on our own, not to mention the hassle of sourcing all the different pieces we’d need.

Keisler’s Tips For Swapping The Tremec TKO Transmission Into Classic Muscle Cars

Because Keisler has a lot of experience with late-model transmission swaps, they’ve come up with a list of common problems and solutions encountered in the process. Here’s a look:

The Problem:
In all cases except for the ’65-73 Mustang, a replacement or modified driveshaft is required, and the slip yoke must also be replaced. This is because the Tremec is longer than the original transmissions by 1.5 – 3 inches, and the output shaft has a larger diameter with a different spline count than stock.

Keisler’s Answer:
Keisler manufacturers all of their driveshafts in-house using a HINES driveshaft manufacturing cell. These driveshafts are automatically pressed together, then robot MIG welded for consistent weld joints. Finally, the driveshaft is 2-axis dynamically balanced, masked, painted, and a certification sticker is applied. Keisler manufactures a line of forged and CNC machined slip yokes to guarantee performance and fit in the classic car tunnels, which are generally smaller than newer cars.

The Problem:
In all cases where a factory four speed is being removed, the transmission crossmember must be replaced or modified. This is because 5th gear adds length to the transmission, thus moving the mount location farther back. In some cases, the original crossmember can be modified and reused.

Keisler’s Answer:
Keisler provides high quality MIG welded, box construction, powder coated crossmembers that mount to the factory frame mounting points and provide rigid support at the correct driveline angle. This is very important. Shortcuts that lead to lowering driveline angle will result in unwanted vibration, vehicle harshness, and premature wear on the driveline.

Clutch Disc
The Problem:
The Tremec TKO uses a 1-1/8 inch x 26 tooth input shaft, well recognized by owners of the Muncie M22 (a.k.a “Rock Crusher”). The domestic automotive manufacturers have standardized on this spec, but four speed owners will need to change the clutch disc.

Keisler’s Answer:
Keisler supplies a range of organic and copper ceramic clutch discs, whereby the clutch pressure plate can be reused.

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