Rim and tire fitment question

Ok, I'm bad with math! I currently have stock Mopar Rallye rims on my 72 Dart. Large bolt pattern, 15 x 6.5 with 4.5" backspacing and 255/60-15 tires on the rear. They just fit. Clearance from the leaf springs is 1/2", as is the inner fender lip, also. I'd like to get Crager SS's 15 x 7 with a 4" back spacing. Will this be an issue??
What about Wheel Vintiques Rallye 15 x 7 with a 4.25" backspacing?? Better choice?
Or do I just stick with what I have and move on with my life! Lol!