71 Demon wheel size/backspacing

To the inside on a dart, with offset shackles, 4.5 or 4.75 is ideal on an 8" rim. The variable is to the outside as the quarters are a little different between the 2 cars.

Best advise is to get out that straight edge and see........

Total clearence
Clearence to springs (which is the first thing you'll hit)
Clearence to inside quarter lip

Decide what clearence you can live with. I'd like to see 3/4" in each side when it's all said and done. Less than that and you might have some occasional scrubbing.