63 Dart: biggest steel wheel without mods??

What is the largest size front/rear steel wheel size I can fit on my 63 Dart without mods? Will a 15x7 work or 14x6 or ??? I currently have 13" steel wheels on it, but I want to put RWL tires on, but you can't get them anymore for 13" rims. These wheels are too tiny anyway so Any ideas???