I think you're right Bill, The Chevy FAM shows 1/2 ton frames as 48" wide at the rear and 3/4 ton and larger as 38" wide. I'll bet that GMC's were the same (probably shared the same frame rails,) with the 100 series long bed, with longer wheelbase being lengthened forward of the front of the rear spring mounts. A unique frame most likely.

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.