74 Duster stock springs, stock 8-1/4 rear

As it turn out I don't care for the Mopar cop wheels I had and have put on my Duster with Cooper Cobra's. To me it looks incomplete, even if I got centers and trim rings.

My Summit makes a nice weld wheel knockoff, 15x8, that comes in a 4-1/2 backspace.
some folks I know who know what they are talking about have said I can get up to a 275/60 in there on that rim. Any thoughts would be appreciated.

Thanks