8 1/4 rear end 2.45 gears!

Thanks, Cass.
I knew that there was a "break" in the axle in the same way that some axles from other manufacturers had. In short, the differential carrier was different based on the gear ratio. The taller the gear, the thinner the ring gear so the carrier/differential had to be built closer to the centerline of the housing. I've read that some GM axles used a thin shim that went between the ring gear and carrier to make things work.
The "break" is as stated. 2.45 to 1 and taller is one carrier/differential, 2.71 and lower is another.
I didn't know about the difference in housings but now I do.
I have a disassembled 8 1/4" axle that came with 3.21 gears and a 69 Dart 4 door with transplanted 8 1/4" axle with a 2.71 Sure Grip. I'm contemplating having a shop swap the 3.21 gears into the Dart axle. I wish I knew how to set up gears myself!