What size tires will fit on stock dart 15" wheels?

so i think I can fit 4.5 backspace steely 15x8's with 255/60r15's on my 1970 dart with a 3/4 inch spring offset on an small bolt 8 3/4 rear end set up... Right?
and upfront, with smallbolt disk brakes, I can fit the 15x7's with 4 inch backspace steelies with 215/65/r15's??? Or can I go wider than that???
I am sorry, I am having trouble with this, thanks for your help...