63 Dodge Dart, now let me see;)

if the current 7" rim has 3 3/4" backspace and the 8" rim has 4 1/2" backspace then as above the inner edge would move inwards 3/4". the issue is it will also move the outer edge 1/4" out (totalling the 1" extra width of the 8" rim) making the rubbing worse. without looking back i remember someone suggestion you have a little machined off the mounting face of the 7" rims you have. 1/4" would probably sort the issue for you. then if needed a size smaller tyre also.
neil.