I would like to add that the rear for that car will be a 10-bolt 8.75" which was the big Pontiac rear 1965-70, after which GM standardized it on with their big cars, specifically wagons and/or 400+ cube engines for 1971. That rear continued into the downsized era on B-body wagons & police cars into the 90's, although general usage ended after 1979 when the 403 Oldsmobile engines & other 6.6 litre offerings ended.

At least that will give you a wider range of potential donor vehicles. The clamshell wagons of 71-76 almost always used a 3.08 gearset in an 8.75" 10-bolt rear. Probably your best chance at a 3.08 is from one of those. Also 1973-77 A-body wagons (only).