4 speed ka-booom !

Build a Dana for it, car is not worth a crap sitting in the garage next season broke.
2th's really, you mods sound like they will cost 3k, time you factor the cost of fixing the broke 8.75 you have.
Find a junk yard 60 out of any GM, Ford, Dodge truck, have it shortened to off the shelf length axes and be done. Not as much money as you think.