The take home should be that a 15x7" with a 3.5" backspace on an A-body 8 3/4 is pushing your luck, especially with a 255. Typically, with a large bolt pattern A-body 8 3/4 in a Dart with the stock spring locations the better bet is a 15x7 with a 4" to 4.25" backspace.
Far more likely that the quarters aren't in the same place. The body tolerances are much looser than the suspension and chassis tolerances. Not saying the perches can't be off, but, it's FAR more likely the perches are where they're supposed to be and the body is not the same. Pretty common even on the same car, when one wheel house has more room than the other.