IMO....The 1965-1972 Caprice wagon, Impala, Biscayne, and Bel Air 12-bolt rear ends will interchange with a '66 Caprice and they use 30 spline axles. The 1965-1972 Pontiac Bonneville, Olds Delta 88, Buick Electra 225 and Cadillac Brougham 12-bolt rear ends and they use 31 spline axles. The 1973-up GM 12-bolt rear ends will not interchange with any 1965-1972 GM vehicles in any way.
That is part of what I found out when I was sorting out a Chevy 12-bolt Positraction for my '66 GTO. I finally gave up and installed a 1965-1967 4-pinion, 3.90:1 Saf-T-Trac for a Pontiac Firebird, GTO,Tempest, LeMans differential 10-bolt rear end.