Wheel to spring clearance

straight edge from brake face to leafspring. deduct 1". There is your max wheel backspace.

That will require a tire that is no more than .5" section width wider than the wheel itself.

yes, you can use spacers for fine tuning the wheel fitment but realize that if you use a spacer you lose on the quarter panel side.
OK, I'm going to go and measure it now. I'm getting 5.5" inches from the wheel mount on the brake drum, to the leaf spring. Wheel mount to tire edge is 4.5 " laying flat on the floor. But on the other side with the wheel on, still have 1.5 " inches to the spring. So it looks like I can do a max backspace on the wheel of 4.5 to 5".