Fattest Tire under a 73 Dart swinger

I know this question is asked all the time but I can't get a solid answer and I dont have the internet speed in Afghanistan to search through a bunch of pages to find it! I have a 73 Dart swinger I was told I could get a 255/60-15 tires to fit on a 15-8 rim with 4.75 backspacing.... do you think I would still be able to squeez it under there with 4.5 backspacing? What sized tire have you been abe to fit without modifications! Thanks for your help!