Backspacing Question...?????

Usually 3.75 is about the least you want.

On a 68 Dart a 245/60/15 with 8 3/4 rear small bolt pattern is centered with 4.25" backspacing.

On the front 4.25 is the best backspacing for a 15x7 rim.

When you get into the 3.5 and 3.75 backspacing in front you might rub the lower front fender lip if you get a tall or wide or both tire.