Just lay a steel ruler across the face of the hub where the back of wheel mounts. Then measure the distance from the ruler to your leaf spring. Deduct 1", and that is your max back space.
Sorry I'm late, looks like the big wheel haters are already trying to force their archaic nonsense on somebody else because they still haven't figured out how lousy their BFG T/A's really are.
at least you weren't just slamming the OP like the usual suspects because he offended your delicate sensibilities by suggesting something other than 50 year old technology.