Even though I have 2 Cutlass' I am more into B-bodies. There is a lot of "mystery" about the different years. The earlier years (65 up to and including 70)had two different rear-end types. 65 & 66 had the "pontiac" type and in 67/68/69/70 had the full-sized Olds 12 bolt(different from the A-bodied Olds 12 bolt.) Nobody makes "anything" for these rear-ends that I know of. Starting in 1971 Olds B-bodies went to the Corporate rear ends. My 71 Delta (455cu.in. 2-barrel) has the corporate 10 bolt 2:73 gears but I have a 1971 Olds 98 (455cu.in. 4-barrel) has the corporate 12 bolt also it is a posi.
I have heard the 4-barrel 455's all got the 12 bolt but who knows for sure but it was true in my 98's case. These rear-ends will interchange up to and including the 1976 B-bodies. Ken