front wheel size for 1968 Dart

A 15x7 is fine, it's really about the widest front rim that's worthwhile with 15's. Backspacing depends on your braking set up, if you've converted to BBP then 4.25" of backspace usually works well with up to a 225/60/15 tire. Too much more backspace than that with a 15" rim can cause interference with brake and suspension parts. Less backspacing than that will limit your tire size because of fender clearance.