I checked on EPC and all transmission castings for M117, 119 and 120 have different part numbers. Your best bet would be to find a junk yard where they have different transmissions and engines sitting around.

I'm not sure this should be developed separately from the flywheel, clutch pack and fork. They will determine the diamenter and depth of the bell housing. One avenue may be Mustang parts mated to a Tremec. The other one would be BMW parts mated to a Getrag.