8-1/4
There are the most popular rear to find in an A body car and the rear was installed in them longer and later than other rears.
The 8-3/4 is a popular unit that can be found at a decent price if you wait for it to come along. Bargin hunting was never my thing. (As well as payin to much for it.)
I forget whom does it, but you could order one from Moser I think to bolt in. But thats the 9-3/4 Dana rear for something like 1300+ bucks.
I got my 9-1/4 for $200 w/a 3.21 suregrip in there allready.