1965 Barracuda dash in a 1964 plymouth Valiant ?

Use the entire dash assembly (metal frame and all) with wiring. You may have to get the engine/front end harness too, or, at least modify it. at the bulkhead connector. There may be a radio difference IIRC....round vs rectangle buttons? The glove box insert is the same between the two though. I'm assuming that the '64 is a manual trans car? If not, what are you going to do about the shifter?
Good point about the pushbutton shifter! If necessary.