Volkswagen Beetle Forum : “Are 64 and 65 doors interchangeable?”

Negative!

well, yes the you can interchange them.  the problem is that in 1965 the window openings on the doors were enlarged.  SO, if you put 65 doors on a 64 car, the window lines won't match.  most people, that don't know what they are looking at, will never know.  if you change BOTH doors, it makes it even harder to tell.