[SS Tim]

Not sure where to begin but lets start with the actual GMC Fenderside/Stepside models.

The shortest or SWB would be a C/K-15704 with a 115" wheelbase. The bed has an inside length of 78.12" and two stake pockets per side.

The middle size is a typical LWB, a C/K-15904 (1/2 ton) or C/K-25904 (3/4 ton). Both have a 127" wheelbase. The bed has an inside length of 98" and three stake pockets per side.

The longest was only offered as a C-36004 (1 ton) with a 133" wheelbase. The bed has an inside length of 108.25" and four stake pockets per side.

If you truely have a 3500/1 ton, then it has a 133" wheelbase and the shorter bedside and step are not a direct fit. To get a shorter overall bed the easiest way would be to bob you existing bed behind the fenders. The best choice is about 10" down to the 98" inside bed length you desired. This leaves the rear overhang about the same as a SWB truck's.
It is not uncommon but I hesitate to advise you to simply cut up a good bed. There just are not many out there.