76 valiant suspension fit 73 dart?

yep provided the 76 has the same brake type as the Dart. Drum brake spindle upper control arms are different than disc brake control arms. they can't be interchanged.

A few years ago I lost a bet arguing this point, so I have some experience in what I'm about to say: The difference in the ucas is that the 73 and later setup used a large ball joint, whereas the 72 and down A body uses a small upper ball joint. It has always been assumed that drum brake equipped cars built from 73-76 used the small ball joint UCAS but many do not! It is quite possible to find large ball joint UCAs on a 73-76 drum brake car. In fact, since I lost that bet with an FABO member, I've never seen a 73-76 UCA That was NOT big ball joint!
I can't confirm that they don't exist. I do part out about 10 A bodies per year and I haven't found any yet, so the odds are good that you've got the correct UCAs even if your 73 and 76 have drum brakes.