Hello-
It will be a turbo EFI engine. In street trim we hope to make an honest 350hp. I know, I know, you asked about torque not hp but I don't have those figures yet 
. Still on the dyno trying to work through tuning issues.
Transmissions don't care much about power... they can spin 9000rpm pretty well and put through lots of power without much engine torque. What transmissions don't like is a lot of torque... given that you have a turbo setup, I'm guessing that you'll have a good amount!
At cruising speed (70mph), I would like to see 2800-3K rpm but that may not be realistic and also I am not that anxious to do top speed runs in a bus even though I have made many improvements to the handling (as much as you can to a brick
) so seeing 100+mph would be very rare and not a big requirement. I really like the 0-60 excitement best.
For that kind of 5th gear, you don't want a 1972-73 tranny.
You really need to know what kind of torque you're putting through...
The 7:31 R&P is weaker than the 8:31 variety, simply by design (the pinion is only 7/8 of the size!). However, given your large vehicle, it may be a very nice thing to keep around...
If I had to choose the *best* scenario for you, I'd do this:
- get a 1974 911 or 1976 912E core
- get a late aluminum 915 core (one with the second, inner annular rib of support on the sidecover) OR just a late, empty case
- get a Kennedy custom flywheel for the T4 with 911 pull-type clutch components
- get an aftermarket gear-reduction starter for a VW (fits the same as the "self-supported" starters)
- buy a shift fork jig
- make a mainshaft rearward nut removal tool (requires a non-plated, deepwell, largish socket, a spare mainshaft helical reverse gear, and a welder)
- take them apart
- inspect everything
- get any extra parts you need (noting that you should use the LATE style of 1st gear syncro/engagement stuff, post 1976!)
- put your masterpiece together
- sell off the extra parts to get back money
Or, if you don't want to do it yourself, then call up someone and ask how much it'd cost. The most difficult job is to correctly set the diff preload, the ring engagement depth, and the pinion engagement depth. Unforunately, when you swap cases and R&P, you have to do all of this all over again.
A simpler solution is to leave the case magnesium... this means that you can skip the job of resetting everything, but you do have a less-rigid case. The case doesn't usually "fail," per se. It simply holds the bearings and shafts in place with less rigidity, so under high load, they tend to wear more.
Get a copy of Excellence and start calling up all the people with little ads... you will eventually stumble across some people with cores lying around for a good price.
Take care,