Other than the vapor recovery system is there any difference between a '73 Duster tank and a '66 Valiant tank?

Thanks!

If the bottom face has the strap indentations to match the (1) or (2) straps for your app then yes they interchange. The ones I've seen (Spectra from Canada) have a checkerboard pattern & fit either group of A body years. EDIT Different A body tanks may or may not hold more gallons than the next one

63-67 have one side-to-side strap; 68-76 have two front-to-rear straps.

Going by memory, the 63-66 have a smaller-diameter filler tube, and the gasket and the tank opening is also smaller, than the 67-up. Otherwise, I believe the 67 is virtually identical to the earlier ones. The strap indentations for the 63-67 are different than 68-up, but at least some of the repro tanks have indentations that will work with either strap setup.

I believe any of them will physically fit up to any A body trunk floor, which is pretty much the same 63-76.

Way back (20 yrs ago) I used either an early tank on a later A body or vice versa & I was relieved to see that the checkerboard pattern on the bottom let me do that. Perfect fit, filler neck, everything. The gallonage may have been different. EDIT iirc it came from the infamous Frankie Mitchell