Backspacing still doesn't make sense to me.

I think the easiest way is to mount a wheel and tire to the axle (that has approx the same overall diameter) and just measure how close it comes to the spring, inner wheelwell, quarterpanel opening, etc. From there you can get a very close approximation. Some tire/wheel stores also have a tool that can actually figure out what size tire/wheel/backspace will work. Or just borrow some different wheels and tires from friends and try them on. On my duster, I have the offset spring package and SS springs. I have 15x8.75 centerline wheels and 265-60-15 MT drag radials. Got about 1/2" to springs and front of wheel opening. Remember though, due to production tolerances both sides might not have the same amount of clearance. Measure,measure,measure,you get it..... good luck