Time for new tires and rims for the 68 Dart

That ruler shows 1.5" doesn't it?

Why don't you do an offset hanger at this time?

Anyhow, if you have 5.25 to the spring, aa 4.75BS plus a tire is going to be tight, and may require a spacer, but it's best to error to having too much backspacing as it is easily correctable.