Wayne, You have hit upon the issue that has been the crux of homebuilt/kit midengine cars since the beginning. There has only ever been one US built production transaxle truly capable of handling the torque of a big block engine: Toranado. In the '70s and '80s there were a number of projects built on the Toranado drive train, but it really doesn't lend itself to the kind of cars you and I would want to build and drive.
You may want to get in touch with Lee Holman as he builds copies of the T44 that was in the MkII. Or at least he tries and also wants big bucks, ~$30K, for one. The last time I talked with Lee he had 3 freshly cast magnesium cases but no one to machine them to accept the gears/shafts etc. If your friend in Orlando is building T44s, you may want to put him in touch with Lee also. (I suspect he is rebuilding ZFs.) There are several transaxles out there that will handle this kind of torque, but none will be cheap: Enco, Hewland, RBT, Ricardo, etc. The RBT was originally in the new Ford GT, but they have chose the Ricardo for production. I am sure that production volume capabilities had as much to do with the decision as anything. Lloyd Butfoy of RBT has been a contributor here and defended his 6spd version of the ZF design quite well. While these are not cheap either, they are probably the most affordable. While I am sure that Lloyd upgrades the basic design, it was able to handle the 500HP/500ft.lb. of the new Ford GT engine.
Good Luck,
Lynn