Starting a new project car and need help with parts interchangeability

If the Barracuda is mostly stock it most likely has the small 7-1/4 rear end. That definitely has to go.
If you had the V-8/Tremec combo running in the Duster then chances are the rear end is definitely NOT a 7-1/4, might be an 8-3/4. You should ID it. The whole rear end will bolt right into the Barracuda as is. If it is original to the car then it is the small 4" bolt pattern. But because the front end has been converted to the later and larger BBP, the rear possibly was converted also. You can easily check by trying to swap wheels around.