I put a b body 8 3/4 in my 80 Mirada. You need the older style shock plates and B body shocks. You don't have to mess with the spring perches at all. Even my pinion angle was still in spec.

B body perch width is 44", FMJ is 44.46, that's less than 1/4" per side, the springs will easily move that much, my housing dropped right onto the springs no problem. You'll also have to buy new spring center bolts... If you remove all the iso clamp stuff like I did your old center bolts will have a funky 11/16 (I think) head on them, you need plain old regular 1/2" ones or whatever size they are. Get your drive shaft shortened and you're set.