OK,
What years Heep had the NV3500 and or the AX15? I think I will start there and work towards what ROR is saying about the Toyota deal. If the r151 is mostly the same as the AX15 it should be easier to find a bellhousing to mate up. The electronics I will all worry about later. We have no such thing as smog/e-check so lights that are on won't bother me too much if I cannot get it sorted.
R151 is COMMON.. super common. and i know for fact that they are tough. i abused (and cared for) one every day for 75k miles.. only time it ever let me down, was when i power shifted 2nd gear one day, and the shift fork retaining bolt broke, leaving me stuck in second gear. only time ive ever heard of it happening tho, and it was a simple $5 fix, and a couple of hours labor to pull and split the trans.
they made MILLIONS of R151 toyota V6 trannies.
im NOT impressed with NV3500/3550 trannies either. they have WEAK syncros.. my cousin has one behind a STOCK 4.3 v6, and even it has syncro issues.