My new wheels won’t fit

4.25" backspace work with your tire\wheel combo without moving the springs. That is on my 67 Barracuda though.
You Barracuda guys are lucky. Duster, Demon's too. You have enough wheel opening clearance for a 8" wheel with a 4.25-4.50 offset. The Darts are pretty tight unless you roll or trim the lip. I run 15x7's on my Swinger.