Thought this info might be helpful.
If anyone knows which 8.5 ,cars/years ( all i hear is GN, but what years?) are compatible( direct bolt in), i would appreciate the info.

Direct 7.5 bolt in list i was given for my 80 Caballero:
Century 78-81
Cutlass 78-81
Grandn Prix 78-81
LeMans 78-81
Malibu 78-81
Monte Carlo 78-81
Regal 78-81
Sprint 78-81

Direct bolt in but the lug nuts are different on:
Bonneville 82-86
Cutlass 82-88
Grand Prix 82-87
Malibu 82-87
Monte Carlo 82-88
Regal 82-87
Sprint 82-87

Jegs and Summit sell control arm kit that you could use to install A-body(?) diff into a G-body.

Yep, I've got an '86 rear from a Monte in my '78. I kept the lug studs metric, I plan on upgrading the front brakes to new stuff soon anyway.

8½" came in 1984/87 Turbo Regals (3.42) and 1984/87 Olds HO/442's (3.73).

Buick, Olds and Pontiac (BOP) rear diffs are cheapoer and are weaker than chevrolet. I'd suggest no BOP Parts.

Its not my Opinion either. I've read several articles that talk about this. I was always under the impression that they were all the same but according to what I've read in many different articles, they are not and are basically weaker than chevrolet specific.