The Audi gearboxes do have someinterchangability, but there are some internal differences as well (pressed vs. splined gears, etc.). I have seen, and driven a 944 that had an Audi diesel R&P in it - something like a 4.7 R&P ratio. It was geared out at Road America at about 112 MPH in 5th! It had 3L 8V stroker (2.7 block + 3L crank) that made 186 RWHP and similar torque. Amazing car - a cost is no object build from the 80's with a floppy disk data system still in it. We de-modded it into a Spec car, and sold the special parts, including the trans.

Charlie, are you building a 16V motor, or a crazy high compression/revving 8V?

The motor will be an 8V 2.5 built to SCCA ITS specs. Estimated RWHP is 175-180. This was a big market for Milledge back in the day when the 2nd gen RX7, Datsun 240-280z and 944 competed for top honors in ITS.
It will be pretty much stock with .020 pistons, very mild cam, custom tuned DME.
Planning on running a 245/45-16 or 235/40-17 tire on a 9" wheel. It will still have spec suspension and body work. I want to be able to convert back to spec if needed for Championships.
This car will mostly be used for enduros in E2. May run GTS2 or PTD.