Here is another option if you are concerned with rust around the weld. Mine is a fleetside but the assembly is similar. I did not want to weld the rear sill to the bedsides on my fleetside so used this procedure. Assemble the complete bed sheet metal and tighten all fasteners exactly how I wanted it. Then, removed the rear sill bolts one at a time and drilled them out to a close fit 3/8 instead of the original 5/16 and tighten a 3/8 bolt in the same hole. Do each bolt one at a time making sure the bedside does not move relative to the rear sill. By doing this I was able to disassemble the bed, paint, and reassemble in the exact same position without any welding. I have driven the truck thousands of miles over the years and the bed sides are still lined up. Your mileage may vary.

Mark