I know I don't have all the info to make a correct response to this post but...
My current 8.5 rear has an open carrier I am looking to put in a posi unit. The car currently has a low HP 400 from 72ish, it has a turbo 400 behind it. The plan is over the next few years to put the correct 400 HO motor with a M21 4sp.
Phew with that said when I crack into the rearend I want to see what gears are in there, when I add the posi I was planning to put the correct 3.90 gears in? This is where my question lies.. 3.90 gears? what do you think?