I've got that size in a differrent tire on my 68. I had rubbing issues until I rolled the fender lips. Now it only rubs if I go over one heck of a bump. Also, the backspacing will help you determine fitment. Mine is 3.75 inches. 4.5 would be perfect. Another thing to consider is that your speedometer will be off.
I just recently got 5 leaf mid-eyes that are supposed to drop the rear 1 in. My thoughts are it will stay the same height and slightly stiffen it because the old springs are probably sagging.