resin, Put a tire on it, any tire. Then measure how much room you have from the edge of that tire. Add that to the backspace of the rim you put on, Then subtract what space you want between the frame and the rim. Remeber the tire may stick out over the rim alittle. this would give you a reasonable backspace, This should work for you, I did it when I rebuilt my camaro and I got the rims to be the right size I wanted.