Quick Backspacing Question

On my 73 Swinger I have 215/70/15's on 8 inch rims and have about 1/2 or less between the tire and the springs, but more than an inch between the tires and the fenders.

The rims are 4.5 backspace so if I put on rims with a 3.75-4 backspace I could run 225's

Don't know if this helps or just confuses more. :D