Safe Tire Size bet for 71 Dart Cruiser

How wide are the rims? And what backspace do they have? What rear axle? SBP still?

All makes a difference. Most of those cars will fit a 245 wide tire, but only with the right rim width and backspace. So if you don't want to change rims we need to know what you've got. Most stock rims are too narrow for anything wider than a 225.