I've had a chance to do some more research with the 9 3/8" rear. This applies to the 1971-75 version, which I run in both of my big block 76 LeSabres. This rearend design was gone by 76, but it fits up to 76.
This started because I burned up another set of posi clutches in mine at the track. It kept wanting to spin the right side only with slicks in the waterbox. Then it would slowly try to engage itself as the right tire got hot and sticky. I would stop and start the burnout while really laying on the brakes trying to get it to lock up, but the damage was done. This is the second time, and fresh clutches only fix the problem for a short time. Seems the slicks in the water aren't providing enough resistance or torque load to energize the clutch pack, even though on the street it wasn't an issue.
So I set out to replace that feeble "S" preload spring in the posi unit. The unit needs much more preload. After I would burn these things up, it would only take about 30 ft lbs to make it slip. Replacing the clutches would bring it back up to about 100 or so, but that is still not near enough for the track.
I went through some posi unit parts I had and found some Eaton posi springs and spring plates. It looked like they would fit if the plates weren't quite as wide. So I narrowed a set and installed them with 400lb springs. The fit was good after modification, and it now took closer to 200ft lbs of axle torque to make the unit slip. But I was wanting more, and I was starting to worry about affecting the hardness of the plates with all of the welding.
So with more digging through boxes of parts, I found a set of Auburn preload plates and springs for an Auburn posi 8.5" corporate GM application. Turns out the Auburn setup is a an exact drop in. Even the crossshaft was identical. In fact, I used the Auburn cross shaft. So maybe there is an actual use for the Auburn design, the rest of which I do not like. The cross shaft hole dia.(as mentioned above) and spring compression are perfect, as if designed for the 9 3/8" posi. The Auburns, with their cone type clutches, use lots of preload, so it was real fun to install. But I got it in and not even 600 ft lbs. of axle torque will make it slip. May need to find some lighter springs, but these are the plates to use.
Here is a picture of the plates. From left to right. Stock Eaton plates/springs, then the narrowed Eaton plates/springs, then the Auburn plates/springs, and finally the stock feeble "S" spring with a picture of it installed.