The 79 to 81 gm a/g body (Malibu, monte carlo etc) rear ends are very close in width, if you can find a donor car and graft the rear subframe in that should work. these cars have triangulated 4 link and coils, ride very nice. I did one in a 51 dodge pickup and it was 530 hp, I installed a pice of plate in the lower control arms and ran a bolt as a poor mans box, basically making it a solid piece and it got awesome traction too.
If you want to easily convert to open drive line 2nd gen camaro rears fit the exact width and the spring perches as well. I think you only have to drill the center hle 1 inch back and install a center bolt.