early A - later A interchange

Most 64-66 A-body stuff will interchange, for example you could bolt a 66 Barracuda front end onto your Dart, or bolt the '64 Dart front end on a Barracuda or a '63 Valiant.

As mentioned, interior stuff will all interchange too depending on 2-door or 4-door etc.

Most mechanical stuff will interchange right from '64-76. K-frames will not because the 2nd gen A-body is a little wider, but you can take lower control arms, upper arms, torsion bars, springs (I think), brakes, rear ends, etc. I've heard of someone taking a Duster rear floor pan and fitting it to his '65 Valiant. I think it's the same, but the Duster one would be wider so he had to cut some off.