I made my comment about the Ranger box because I had a 2000 Ranger with a box at the same time I had a 50 3600. The drop-down portion of the Delta alum box was about 1-1/2" too wide to fit in the Chevy, as I recall. The overall width was almost exactly the same as the width across the outside of the bed rolls (tubes). So, the mod I suggested would make it fit the Chevy and not look too bad. I lacked the tools or skills to make the mod. Your mileage my vary.
Back in the day when step-side trucks were still being made, almost all the box manufacturers sold boxes for that application, and there are probably some still out there sitting in somebody's barn or garage...
