I called Modern Drivelune and left a message and I'm still waiting to hear back. Here is something I found on line. I hope this helps..
The advantages of overdrive need no introduction. These days, driving your Ford Muscle machine on the freeway without overdrive means revving the heck out of the motor or running late 70's smog era low numerical rear end gears. Neither choice is attractive for gas mileage or performance, or what we really want - both.
For this car, a 1971 Mustang originally a 302 car and now packing a 572 stroked 460, the existing C6 was spinning at well over 3000 rpm to stay with freeway traffic, even with compromise 3.50 gearing. Add the fact that the streetable 2400 stall speed was well below optimum for a drag strip launch, something more flexible was called for. The preferable OD manual transmission rated to handle the torque of a stroked 460 was the six speed, Viper spec, Tremec T56. A couple quick calculations showed that the ideal rear end gear with a 28" tall tire and a 6200 rpm shift point would be 4:11 to 1. With the T56's .50 6th gear the final drive is close to 2:1, meaning a little over 2000 rpm at 75 MPH. Talk about having your cake and eating it too.
The only problem with the T56 swap is that Ford only offered the 429 engine in one model year of Mustang, 1971. Therefore there is a lack of instruction and guidance on performing the T56 graft into this year Mustang. That is, until this article.
Myth Busting
Myth #1 - A manual transmission means weight savings.
We all know the Ford C6 is tough but heavy. How heavy? On the authors' handy physician's scale the C6 with converter, flexplate and fluid weighed 218 lbs. The T56 with fluid weighs only 128, but add the NHRA mandated scatter shield (55 lbs.), steel flywheel and twin disk clutch assembly (48 lbs.) and the stick tranny is actually heavier than the C6 by 13 lbs. You can save a little weight during the conversion using an aluminum flywheel.
Myth #2 - Mounting a T56 behind a 460 requires a custom adapter.
For this project, the turning point was the introduction of the McLeod modular steel bellhousing. This meant a straight forward bolt up of the T56 to the back of the 460 and the avoidance of custom machined adapters.
Myth #3 - The C6 is huge, if it fits so will anything else.
While it's true the high bellhousing mounting pattern on the 429-460 and the M family of Ford engine takes up a lot of space, but the automatic
While comparable in length, the T56 case has greater bulk, requiring transmission tunnel modifications. transmission tapers off quickly. The manual's six beefy forward gears and the internal shifter rail takes up more room front to back and adds height back where the vintage transmission tunnel really begins to narrow. Also complicating the issue is that the transmission mount is six inches further back than the location used by the older autos and sticks in vintage iron. The case size stays large back through the sheet metal tunnel brace common to earlier Fords.
The easy fix is to tilt the engine a bit more to get the T56 under the tunnel, but this would mean reduced ground clearance for the headers and transmission, as well as bad driveshaft angles resulting in vibrations. The other issue is the location of the internal shifter. There are two shifter pads on the T56 but both tend to be forward of the typical pony car or mid size Ford shifter location. There are now shifter extensions which add a lever on the tunnel with a link to the actual shift lever, but these would look funky in a vintage muscle car and preclude using a factory console. The other choice is to cut a new hole for the shifter, but again this leaves no room for the factory console and a knuckle busting long stretch for third and fifth gear.
The ideal solution was to set the engine back 1.75" and down .75". This sounds worse than it is as in a '71 Mustang the engine has plenty of room to move back without touching the firewall or moving vital components. It does require some surgery on the tunnel brace but it puts the shifter in the original location and made the drive shaft angles almost nonexistent.
One step at a time
After putting the wheels in motion and waiting for the first production run of the new modular bellhousing to go through quality control I was faced with the decision to do the job of fitting an untried combination from under the car or going through the hassle of pulling the engine and mocking up the install out of the car.
