Are any of the part interchangable? I have several tail shift trans in my "collection and I'm wondering if they have a use, especially since my sideshift may need some attention in the future...

Gears, synchro, bearings Ring and Pinion, side cover middle bearing section. Speedo Drive Angle. I'm sure there are more.

Other parts not really, but they do make nice lamp posts, paper towel holders, paper wights, or just a general tool when you need a large punch for something.

Shifter rods and shifter forks are unique to each setup.
Most of the rest can be swapped over.
Intermediate plate is also different because of the shifter rods.
Tail cone is obviously different.

I believe you can even reuse the case, just not remembering what needs to be done for the side shifter housing....

The only intermediate plate that's different is the one for a 914/6, and that only because the throttle bell crank is screwed into the side. The only case mod is replacing the studs for the shift console, IIRC. You should change the nose cover, but you don't have to do so.

The Cap'n