I am running 15x7's on all four corners of my 69 Dart. My wheels have 4 1/8" backspacing. You just have to be careful not to go too big with the tires.
Your Dart is absolutely beautiful. Im looking to put 15" on my 383 GTS but afraid not to get the correct recipe. What is your tire size? Are you having any issues with rubbing front or back?