I've actually been thinking about trying to build one myself. Instead of a carbon fiber monocoque just a tube chassis with fiberglass or similar body pieces and a VH45-TT with, most likely, a Corvette T56 transmission setup (its a regular 6-speed RWD type transmission with a differential bolted to it to make it a transaxle). They can be built up quite heavily too and I think the length of that setup would be closer to the Xtrac 6-speed used in the R390 (I have no measurements to confirm that though, just going by pictures its the closest thing I've thought of yet... next to actually finding an Xtrac 6-speed )
Most transaxles I've found, say like the very popular Porsche G50, have the outputs near the bell housing. Going by the pictures I've found of the R390, the output are closer to the rear of the transaxle. Here are all of the pictures I've been able to collect so far:
http://www.machzracing.com/R390
Be sure to check out the 1997 Technical folder... that's where a lot of the under body goodies are.