ring and pinion interchangable models

In general, the ring and pinion from Pontiac Tempest, Firebird,& Grand Prix from 1964-69 will fit the 1965 Olds A body BOP axles. Within Olds, generally, the A bodies from 1964-67 will interchange. 1967 was a transition year for Olds, where some "O" axles were used. These will not interchange with the BOP. For some, there are some interchanges with Buick, but not as much as the Pontiacs. There were three differential carrier cases used in the 1965 F-85, depending on the ratio. This was because of the physical sizes of the ring and pinion gears. One was for 2.56 and 2.78; another was for 2.93, 3.08, and 3.23; the third was for 3.36, 3.55, 3.73, 3.90, 4.11, and 4.33. The 2.56 was not used by Olds, but will work in the axle. I have replaced the ring & pinion years ago in mine, to get it better as a highway cruiser; I didn't concentrate on 1/4 mile competition. Used gear sets work fine as long as they are in good condition and are installed correctly. Fuel economy increased significantly. If you tell me exactly what you currently have, and what you want to go to, I'll give you the exact interchange information from the Hollander listing. Keep in mind that "higher" and "numerically higher" are opposites for gears and are sometimes confused.

If you have a 3.08, and would like to go to the same or a 3.23, you should be able to reuse your same carrier case. As there was some impact involved, check all parts to be reused for damage. Damage to the pinion would be revealed in a close examination, including rotation. Either of those two sets should be available from salvage or individual sale on an on-line auction, swap meet, or advertisement. Here are the sources as provided by Hollander for that carrier:
2.93, 41-14 teeth, part number 9775245
3.08, 40-13 teeth, part number 9775246
3.23, 42-13 teeth, part number 9775247
Firebird 1967-68;
Firebird & Grand Prix 1969 w/R. H. thread ring gear bolts;
Olds 1964-66 Jetstar 88;
Olds 1967 330 engine & “P” axle;
Olds F-85 1964-65;
Olds F-85 1966-67 “B” or “P” axle;
Tempest 1964-68;
Tempest 1969 w/R. H. thread ring gear bolts;
Buick LeSabre 1965-67;
Buick Special 1964-67;
(The Buick gears have different part numbers from the Olds-Pontiac, but are supposed to fit.)
Unless the “new” gears are cut the same as the old ones, you may need to change the shim(s) on the pinion and/or the sides of the carrier. If the carrier bearing pre-load was correct before, then the sum of the left and right shim packs should remain the same for the new installation, but the left to right distribution may change due to the change in gears. Try to get the pinion depth set first. Then check the backlash between ring and pinion gears for the new installation. As a final check, do a “pattern test” of the gear tooth contact for the new gears. If all is within specs, and cleanliness and proper lube have been observed, you should have a good axle. You might also need a new pinion seal and “crush sleeve” if compression will be less than where it is currently on the sleeve. If you are a bit “rusty” on the process, read a manual or ask questions. There are a lot of knowledgeable folks on these forums. Proper care will result in success.
Also, if you change ratios, you will need to change the speedometer driven gear, probably one tooth on a manual trans, and two teeth on an automatic.