its not uncommon for it. Tolerances back in the day were horrible. I've seen this before and much worse in some cases. Heck, I measured the distance between the rear axle and the gas tank on two identical, unrestored/molested A bodies of the exact same year with the same options and there was a 3 inch difference.
The gas tank is a terrible place to measure from, the trans crossmember is probly the best , for both ends of the car. the gas tank can be manipulated quite a bit !
