We were asked to look at doing a transaxle for the GT40 kitcar market and/or the ford GT race cars. We worked on a design using a Ford 8.8 axle and our Muncie based geartrain (essentially a clean sheet design of a McKee transaxle). Heres some stuff we found out along the way.
The G50 works but you have to 'flip' the internals and replacement parts are expensive and sometimes not redily available. Theres also an Audi transaxle that some people use for both the GT kitcars and as a Pantera replacement. The problem with a lot of the modern manual transaxles are they're either too small in terms of strength or their gear spreads are ridiculous for performance (a small V6 for example needs a 3.50 1st gear and a .75 OD First gear would be useless in a track application, so why carry the mass if you dont have to) . Theres also some minor issues for shifting it (rods vs cable shifter) and packaging.
Anyway; the answer isn't far away for you; Use a V8 Archie Fiero kit. Use the 'Muncie' manual gearbox version; they can take a pretty hefty amount of power. And, V8 Archie has kits for 3800SC; V8s and Northstar V8s.
How to Build a Hybrid MY8/M17 Muncie, and the Differences Between the Two - Pennock's Fiero Forum
Now get the sawzall.