I just read through the entire build, pretty cool.
Have you got the tranny issue figured out? If not, I would consider going with a Jeep T-150 3 speed ('76 - '79). You can do it with all factory parts, and they're relatively inexpensive. The factory bellhousing used with the AMC 150 ci. 4 cylinder ('82-'86) has the B-O-P pattern you need. I believe it was available with a T-176, also matches the T-150. I 'THINK' the T-4/T-5 version has bosses cast into the BH, so even the that version would work. Sounds like you have a line on a Dana 20. It's about the same length as the T-90/D18 you already have, and even with the HP rear axle, driveline length is a premium. The adapted 4 speed is going to be longer, even with an SM420. Of course, it won't bother me if you ignore my ramblings.