what size wheel for 72 Dart?

be careful with that, my duster had air shocks in the back when we bought it in '91, and both shock mounts on the body side were utterly destroyed and had to reweld/reinforce that area. the shock mounts were not meant to carry body weight, only transient loads from dampening.
But Dusters/Demons and Darts/Scamps are 2 different animals. Dusters/Demons can handle a 8" wide wheel without issues.