what size wheel for 72 Dart?

Sure as far as backspace goes, with the 7's on mine, i can't get my fingers between the lip and the tire on the outside and may be 3/4" clearance against the spring on the inside. With a 8" wide on the rear of these cars, I can almost guarantee rubbing or tire cutting. Each to his own.

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sounds like you'd want another 1/4" backspace (so 5.25" or so for an 8" wide rim), that would shift the tire inboard another 1/4"