Rear window regulator 1968 barracuda

There are 2 gears. The small bearing like part where the winder handle goes is not exactly interchangeable but internal parts from a late model can be used to renew the original. I did this to my 67 driver door regulator using parts from a 75 model 4 door rear window regulator. If the larger half moon shaped gear is worn out, I honestly dont know how much interchange can happen there. Have you looked at how that part is attached to the regulator? It wouldn't be easy task. Even fastback and notchback B'cuda have the winder handle in different locations. I dont know if that means their large gear is different.
In the end, It might be best to just make a parts wanted thread. Rear window regulators never saw nearly as much wear as drivers door regulators. Rear window assemblies do have some plastic rollers and stuff in them and some owners would just fight/overpower rather than repair. I'll assume that is what happened to yours. Plastic parts failed, then metal parts eventually failed also. A good used regulator is out there. You may still need to hit up RT Specialties for the rollers. @barracudakid