Depending on how wide the tires are now and how much the rear was narrowed, you may have trouble finding a wheel/tire combo.
Things you may want to think about..........are you going to run road courses? is going to a bigger rim going to interfere with fender clearances both in front and rear?
Don't take this the wrong way , but I personally like the look of an old Willys with the fat tire look, like an old drag car.
I think you may find it will be somewhat costly trying to change the car to run those bigger rims.