IMO the rear shock support/bracket wasen't designed to hold up the weight of the car. A better way is to get rear springs that are arched the way you want them.
I got mine that way from Spring n things( ESPO )
www.springsnthings.comYou can order them plus 1" , plus 1 1/2" etc....
Lots of members have got there springs here. My car had air shocks when I bought it. ( it's a 70's thing) this was the first thing I removed.
I belive the axles are all the same. no difference big block vs small block.
If your going to build a 8 3/4 for strenght you might as well step up to a Dana 60. Thats next on my list.Dr Diff is 2K everything new for a Dana 60, you can use the brakes off the 8 3/4 on the Dana. I have a 440/727 and it has held up fine so far ( just don't use slicks) but I'm building a 451" stroker and the 8 3/4 .... well I don't want to push my luck!
Hope that helps?
Brad