It is all a question of torque. Trannies and rear ends are rated for a certain maximum torque load. Exceed the rating and you risk something breaking. Lube oil can't save you from that.
I'm in the same boat, sort of. I have an old 88 IROC that I want to restore and beef up. It has the stock T5 tranny which is rated at 300 ft-lbs of torque. The stock LB9 motor puts out 290 ft-lbs. If I replace the 305 with a 350, or do any mods to the 305, it will go over 300 ft-lbs and the tranny will be in danger of failure if I open the throttle wide. I'm faced with retrofitting in a T56 which is rated 450 ft-lbs, or a Tremec which can handle 500 ft-lbs.
For your hypoid rear end, I'd look at a cocktail of redline 75W90 plus some lightweight shockproof mixed in.
Have you considered the universal joints?
What clutch are you running? Centerforce dual-friction?