Be careful when using charts that list "rear end widths" and pay close attention to where the measurements were taken from. They might have measured from drum to drum or from axle stub to axle stub or from the centerline of the rear tires (track).
I suggested the Gen I Camaro after finding the TRACK of the '48 Pontiac and the TRACK of the Camaro and the fact that the Camaro is leaf-spring suspended, just as in your car. I'm not saying that the Camaro is the only choice out there. As was suggested, get all the measurements off your car, then take a tape measure to the various boneyards in your area and begin your search.
Also, if you need to tweak the track a little to use a taller or wider tire on the back of the car, wheels are manufactured with any range of backsets.
Wheel tech - Wheel offset clarified