another wheel and tire fitment thread question

Ok here is the situation I have a 72 dusted with a 8 1/4 rear believe to be from a 73 duster. It has14 x5.5 rally wheels with 4 inch backspace and 235 60 14 . The tire is only about half inch from hitting springs. I have seen a lot of threads with dusters fitting a 275 60 on without spring relocation. What backspace are u using and also I wanna run a 15 tire with 7 or 8 inch wheel. Springs are in stock location. How wide of tire should I be able fit with what size rim and backspace. Thanks