Swap 65 Valiant grill for a 65 Dart grill?

Jim 68cuda said

The Canadian 63-64 Valiant Signets were actually the reverse of what your plans are. They had the Valiant front end on the Dart body

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That applies to all '63-'64 Canadian Valiants, not just the Signets.

And yes, the OP would need to install a '65 Dart front end assembly ("front clip") if he prefers a '65 Dart grill. The only grills that will fit a US/Canada '64-'65 Valiant are as follows:

-US/Canada '64 Valiant
-US/Canada '65 Valiant
-US/Canada '64-'65 Barracuda
-Australian AP6 Valiant (that's this one).