IIRC the 66 Dart has no front valence as i think the bumper covers that area. Maybe someone who has one - @toolmanmike - can confirm. If so You might be able to get them to fit on the front by removing the 65’s valence. BTW the 65 and 66 turn signals are not the same even though both mount in the bumper. For the rear i doubt they’ll interchange - there is the bumper indentation in the 65’s quarter panels. Like the front there is no rear valence on the 66 so even if you could get the 66’s to fit, you’d have to remove the 65’s valence.
I have a couple of 65s and 66s.
The rear quarters are actually the same. The 66 rear bumper was made to fit over the indent in the quarter - they put a square bumper on a car designed for a round one.
My guess is the rear goes on perfectly.
The front fenders on a 65 are curled under at the front and you may end up with a gap at the bottom corners between bumper and fenders (possibly may need to trim top corners of bumper) I’d bet they both go on over the valances and with the 65 brackets.