1970 Dodge Dart Ttire Issue

i have American Racing Outlaw II's on the back of my '71, they are 15x8. i have no idea what backspace, they were already on the car when i got it. the tires are 255/60-15 BfGoodrich Radial T/A's. they fit with no rubbing either, close to the leaf spring and the fender lip, but no problems yet. i do plan on later changing them out for 15x7 steel rims with the same size tire. i'm going with the black steel rims and dog dishes for awhile.