Easy done. The later ones were generally all the same. The engine mounts share the same bottom stud, but you will need to add some plate and a captive nut to bolt the top of the V6 mount to. After that, it all just lines up.
Actually, as you have all the GTV6 stuff to bolt in, it should be really easy. The big concern I see will be the wiring looms. If I were doing it, I would be inclined toward taking the looms out and putting the GTV6 one in the other shell before doing the mechanical install.
I think you will find that if you have air con in either and you are going to re-install that as well, all of the firewall penetrations will be there as well. The unused ones usually just have rubber plugs in them. The engine bays on all these cars seem to be the same for the 4 and 6 cylinder and sedans and coupes.
Certainly the ones I've worked on and wrecked here in Oz all were. There may be exceptions of course, but Alfa tended to re-use the same engineering a lot. (Saved a lot of money).