Rear spring hangers

[Seanext]

I have what I was told was a 1972 short bed 4x4 frame. It came with correct front and rear axle numbers for that year. The rear had 54 inch springs. When I started looking at the mounting it was clear that it was originally 56 inch. The spring hanger were still riveted on so I know they had not been moved as there was no extra holes in the frame. I held a 52 inch spring in place on the front hanger and the spring mount for the front is about a 1/2 inch to wide for the spring. I looked close the hanger is still riveted no extra holes and the spring hanger is not bent. I have measured all the cab mount points and bed mounting points and it is correct for a 72. Did they use a wider spring in the rear for any reason? Was the GMC rear spring wider? I am scratching my head trying to figure out what is correct.