Will a 15" wheel fit on my 63 Dart

Plug your tire size into the fields on this page. Very helpful. Backspace is different according to width for factory wheels. I've got 195/60R15s on the front of my 64 Barracuda with the 73 up disc brakes. The wheels are 15X6 with 3-1/2 backspace and they work nicely. You've got a fair amount of room to play with in back so if you keep the diameter near stock your speedometer will remain fairly accurate.

http://www.rimsntires.com/specs.jsp