Mopar in a Mustang ...

Probably wouldn't be all that difficult is you have fabrication and welding skills. But seriously why bother. A fastback that has a ford motor and runs and drives would be worth a lot more and could be drove for a while and sold to help finance your mopar project. A good running 302 is pretty easy to find. Personally I am not a big fan of cross brand engine swaps especially on older vintage iron.