The weak link between these two items is the stock 147,000-mile T5 transmission. While it has survived several mid-11-second passes, it's showing its age with a healthy crunch when shifting into Third gear. The stock T5 has never received the full brunt of the supercharged 331 either, which may be why it has survived until this point.
That being said, we thought it wise to upgrade the box before it disintegrates into an unusable mess of metal, however, we weren't looking at a rebuild. Instead we opted for Transmission Technologies Corporation's T56 six-speed gearbox. TTC supplied us with the Tremec T56 unit, though consumers will have to go through one of its distributors, such as D&D Performance in Wixom, Michigan.
D&D has perfected the six-speed swap for all late-model Mustangs and offers all of TTC's Tremec transmissions in addition to numerous upgrades for each one. The Fox-body version of the T56 has been fitted for a mechanical speedometer (both F-body and newer Mustang pieces utilize an electronic speedo) and an integrated bellhousing that bolts right up to our Ford small-block.
The company can also provide a new crossmember and driveshaft for your application, as well as SFI bellhousings for racing. D&D's Don Walsh Sr. was extremely helpful in selecting the right parts for our installation.
The actual transmission swap was fairly simple, with only minor modifications needed to get the job done. The T56 weighs in around 122 pounds, while the T5 comes in at a scant 75. We scrubbed off a bit of the difference by exchanging our Lakewood scattershield for the integrated aluminum unit, and the extra torque capacity (400 versus 300) of the T56 was well worth it.
The Clutchmasters FX300 clutch has served us well even with 580 rwhp. We had Clutchmasters send us a new flywheel insert for its aluminum flywheel, but upon disassembly, we found the flywheel and clutch surfaces to be in excellent shape and in no need of service. The T56 we used employs a 10-spline input shaft just like the T5, so we didn't need to change the clutch disc, either. Just the same, the entire assembly had to be removed to swap out the engine plate. While we were at it, we installed a new rear main seal. | 
We contemplated giving the Tremec short-throw shifter a go for a while, but we opted for this Power Tower unit from Pro-5.0. We've had nothing but great results from the billet Pro-5.0 pieces, and its T56 unit has proved to be of the same quality. | 
D&Ds new T56 crossmember (lower) moves the mounting point rearward. D&D's swap package also includes bellhousing bolts, a speedo gear and cable, and a transmission mount. The supplied mount is rubber, but the crossmember does without the outer rubber bushings, which is probably a good thing since the T56 is a bit of a tight fit in the tunnel, and you don't want it moving around too much. |