FWIW, way back in 1974 I purchased a used 69 GTO that I know was never wrecked, as I stripped the body down to metal for a custom paint job. At the time I had L60 14 tires on the rear. They also rubbed the sidewall on one side because the lip on the quarter panel was closer on one side.
Just as you said about 1/2 inch difference side to side. Having owned a lot of Pontiacs with as much rubber as I could stuff in the rear wheelwells, it's not at all uncommon for one side to be a little closer than the other. Having lived through the late 60s early 70s when these cars were fairly new, it wasn't unusual at all to see a variance side to side, when trying to stuff the biggest possible tire under them.
Two options were rolling the inner lip up into the wheel well, or grinding some of the seam away. Just a variation in factory parts sometimes will give you different clearances, the frame and rear axle could be fine, just slight variances in assembled body panels.
My 73 T/A was closer on one side than the other too, just trimmed the edge back for enough clearance.
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Brad Yost
1973 T/A (SOLD)
2005 GTO
1984 Grand Prix
100% Pontiacs in my driveway!!! What's in your driveway?
If you don't take some of the RACETRACK home with you, Ya got cheated 
  
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