stock mopar rims are approx. 1/4" positive offset, so if you have a 7" rim (8" overall width) it should have 4.25" bs.

a 5" rim (6" overall width) will have a 3.25" BS. a 5.5" rim (6.5" overall width) will have 3.5" BS, etc, etc.

if you want 15's, a dippy or 5th ave is your best bet if you have the large bolt pattern. if you have drum brakes in front it will be a 5x4--the small bolt pattern. most ford rims will work on the large bolt pattern if you open up the center register .060 or so. small bolt pattern, newer 5x100mm can be made to work, at least for rolling the chassis around the yard.