66 Dart station wagon vs 66 Valiant driveshaft

'66 Dart wagon + Valiant all models + Barracuda, with 7-1/4" rear and automatic: driveshaft is 51.61" long. You don't need to match this precisely, you've got at least 0.75" worth of plus-minus without making any problems.

But as it happens, '67 Valiant + Barracuda with 7-1/4" rear axle and automatic, 3-speed, or 4-speed trans also uses a 51.61" driveshaft. By '69 the figure is 51.43", still completely compatible. In '72, the figure is still 51.43" and that applies not only to certain 108" wheelbase cars but also to certain 111" wheelbase cars. By '74 it's 51.44".

So no, it will not be difficult to get hold of a driveshaft for your car. Aside from '66 Valiant/Barracuda + Dart wagon, any 108"-wheelbase '67-'76 A-body (that is '67-'73 Valiant except Scamp, '70-'76 Duster/Demon/Dart Sport) with the appropriate transmission and rear axle combo, and certain '67-'76 111"-wheelbase A-bodies (Dart, Scamp) with the appropriate transmission and rear axle combo will donate one of the correct length, and there very well might be compatible driveshafts from certain F- and M-body cars as well.