Per the info from the GM heritage site, the 7.5' bed is the same width as the shorter bed, 50 inches inside measurement. Add to that the thickness of the metal, 1/16" per side and the dimension between the fenders would be 50-1/8". Probably not enough to make a difference if you just went with 50 inches.

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.