WIDEST TIRE?

As roccodart440 mentioned above, wheel offset will be critical. Stock offset wheels with fat tires tend to hit the springs first, I believe.

IIRC, the stock Dart/Scamp rear wheel wells with stock spring perches and non-rolled fender lips can handle something like a 255/60/15...which would be a gigantic upgrade over what you're running now.

Do you have a wheel in mind?