Gordon,I think what you have is a bed from the Power Giant trucks. I have two of them, one from a 58 W100 and one mounted on a 59 W100. They must have made them narrower to to keep the proportions in line with the 6 1/2 ft sides? That's the only reason I can think of. The fenders are the same from 53-85 on Utilines though. Hope they go back to the old forum set up soon. This reminds me of the "new and improved" eBay Motors they kept trying to force down our throats. If it ain't broke don't fix it.
Bed Sides of my 61 to 71 are all the same. 79 inches on all then. The same is true with the 79 inches on "the unknown one" which I just mentioned. Ron, Your 1958 and 1959 is a helpful start/answer to this question. Gordo