"65 Barracuda fenders on a '65 Dart

Rockerdude is correct. You can put any '63-'66 A-body front end assembly on any '63-'66 A-body, and the body lines will match up just fine, but the mix-and-match interchange is narrower. '63-'64 Dart fenders will swap, as will '64-'65 Valiant/Barracuda fenders (possibly also '63, not sure). That's about it for single-piece interchange. See Doug Dutra's Dacuda here.