FWIW, I have 15x8 with 4.5" back spacing and 255/60 tires on the back of my 68 Barracuda. I am about 1/2" way from the tire rubbing the spring but about 1 1/2" from the tire hitting the wheel well to the outside.
Using an 8" wide rim with 4.75" back space rim is going to put you at 1/4" away from the spring which could allow the tire to rub the spring during cornering.
If I could do it over I would prefer the 15x8 to have 4.25" of back spacing.
From your user name I am assuming you have a Dart. Dart wheel wells are not a big as Barracuda wells so you may have other clearance issues.
