Change a '64 Valiant to '65

That would be difficult switch because you'd have to graft the rear tail light area from a 65 on there. The front would be an easy switch, just fenders, hood and grille change. The rest of the trim is an easy fix. I'm partial to the simplicity of the '64 myself, but that's a personal thing.

Another potential concern about a '64-'65 conversion would be the core support has a smaller hole in '64 for the radiator. The '64 had a 19" radiator, which was inadequate cooling for the freshly introduced 273, so they enlarged it to 22" in 1965.

For that reason alone, I'd recommend trading your '64 for a '65 to start from, or make peace with chopping out the core support and grafting the rear taillights.