I agree with the rear axel hum'n... my is (good thing I found a 4 lug 8 inch

) the proper level of fluid helps alot, my pinion is leaking but I suspect from the 60+ dyno pulls I've ran to tune and try different things.
I think they can handle quite a bit before total catastrophe, mine is only showing some failure in the pinion except the axel bearing I spun a few years back. and I'm pushing 136hp to the tires... maybe more if I replaced it...
for now, blow up the 7's it's a cheaper learning curve than an 8 inch, but I doubt the 8 will blow up behind our sixes... unless forced
anyways, IMHO, build the car from the gound up, then go for the power plant, you don't want to tear back into it once it's going