With an 8" wheel you'll need 5" backspacing. Depending on how the car is screwed together, that could be too much. But easily fixed with a small spacer. Unless you can really accurately measure, that's as close as you'll ever get.
I can't say a 255 tire will fit, it's very doubtful on most Darts without modifying a lot of stuff. Hell, I couldn't even get a 245 tire on my '69 no matter what wheel I used. If you really need to go that big, start looking at spring relocation kits, offset shackles/mounts, and trimming the fender lips.
The 7" fronts should have at least 4" backspacing, up to 4.50" will work.
I'd down size the fronts to 225/60-15, the 235s are bound to rub somewhere no matter what you do.
