Nv3500 vs ax15
@dartslantsix and
@75slant6 are/were running an Ax15. Dartslantsix had his for sale several months ago.
Either way, they should both have some idea of how they work in a car.
Also, the AX15 and R154 have the same bellhousing bolt pattern. As does the AR5/MA5. So you could start with an AX15 and then move to something more "car-like" if you wanted. Only caveat is the R154 seemed to have gotten pretty expensive over time.