Found this link on an older post...guess I'm answering my own question:-)
http://442.com/oldsfaq/ofdif.htm#DIF...ial%20Swapping
Swapping
Gears are available for most every differential. But buying and installing a bunch of parts can get expensive - fast. It would probably be a bunch cheaper in the long run to find the rear with the gears and anti-spin that you want.
Interchange Guide
From 64-66 to 64-66 Model(s) GM A-body - Cutlass, Chevelle, LeMans, Skylark = bolt in.
From 67 to 64-66 GM A-body Lower spring perch is different, requiring the use of the 67 springs.
From 68-72 to 64-66 GM A-body Axles are about an inch wider than the earlier ones and also have the control arm mounting locations in a slightly different place; the axle will physically bolt up to the early car but the pinion angle will be wrong.
This table works in the opposite direction ("year to" to "year from") as well.
[ Thanks to Joe Padavano for this information ]