'63-'66 Valiant/Barracuda will be the same and honestly from looking at the two, I don't think there will be any difference between them and what you would need for a '63-'66 Dart.
Truth be told, I don't see why this won't work on an early-a as is. It's been a few years since I've had anything post '66, but I don't notice any difference, or at least not one I can remember.
Let one of us early-a'ers try one out, quickest way to sort it out. You should be able to find someone in the TMC willing to help out, if you don't let me know and I'll see about making a trip down.
BTW, how much were these beasts going for again? I was thinking it wasn't too awful much. I might would be willing to just bite the bullet and get one, the gas cap I currently run already leaks and doesn't look anywhere as cool as a flip top.