Modified Mustangs & FordsHow To Drivetrain
Ford Mustang Overdrive Transmission Swap - Swap To The Future
Upgrade Your Classic Mustang With A High-Tech Overdrive Transmission
In addition to the overdrive transmission itself, there are several other parts you should upgrade at the time of installation. A modern diaphragm clutch like those offered by Centerforce is at the top of the list. They offer a tremendous advantage in clamping force and efficiency. Trust us; they will make you forget all about those three-finger units of old. The clutch fork will also need to be upgraded to match that high-performance clutch, and you might run into some pedal-linkage adjustments on your path to adding another forward gear.
Actually fitting the trans to your engine will require the right bellhousing adapter, something you can source from many shops. We know JBA makes several for the classic Mustangs.
You will need to pick up a good shifter for your new transmission. Luckily, the 5.0 Mustang market has forced dozens of companies into this market, so take your pick. PAE uses a model from Hurst because the company also manufactures chrome shifter handles and vintage shift knobs that match the interiors of the earlier Mustangs.
Also, a new driveshaft will be needed on most cars. At the minimum, you will need a new driveshaft slip-yoke that will allow your current driveshaft to fit the newer transmission's output-shaft spline count.
Remember to keep an eye on driveshaft alignment. This can get out of whack if the transmission doesn't quite line up correctly due to some interference with the body or other component under the car. Another common problem is the wrong crossmember and transmission mount for your application.
As a general rule, the Tremec line is beefier and stronger than the Borg-Warner stuff it was designed to replace. As such, you will run into more clearance issues with a Tremec. As a general rule, PAE classifies the swaps like this: A Tremec T5 is pretty much a bolt-in as far as fit. A Tremec 3550 or Tremec TKO takes minimal modification, which includes moving the shifter bezel in the floor approximately one inch toward the passenger side of the trans tunnel. This means you will have to massage the floor of the car. A Tremec T56 is a serious modification which will require moving the trans-tunnel crossmember up to retain the correct driveshaft angle. You'll have to clearance the floor as you would with other Tremec units, but you will be rewarded with a sixth forward gear.
As for the year of Mustang involved in the swap, the '65-'66 models are the tightest with the '67-'68 cars offering a bit more working room. On some '68s, PAE can get away with retaining the stock driveshaft. Again, this depends on the car and application, but you will need a different yoke to go from the old transmission to the new.
Paul sums up the installation process: "In the world of modified Mustangs, I don't consider them big hurdles, just items that need to be addressed. Some of these are driveshaft length, exhaust clearance to the trans crossmember, different methods of clutch engagement, and shifter appearance. Every aftermarket parts manufacturer designs its part to fit a bone-stock car. Once you stack up a combination of aftermarket parts not related to the tranny install (such as exhaust, headers, subframe connectors, and so on), you sometimes find interferences. What makes for a great result is finding a shop that is knowledgeable enough and experienced enough to pay attention to all the little details that result in a rattle-free installation.
Stepping up to a modern, overdrive transmission is one of the best upgrades you can do to your classic Mustang. While we've outlined a few problem scenarios here, remember to consult all of the manufacturers before starting your project. Question other Mustang owners on similar transmission swaps. In the end, the work and planning will be well worth the reward: a quicker, more efficient Mustang that is fun to drive