There are all sorts of truck tool boxes out there. Your bed is about 50" wide inside, a good fit for a 48" box.
Try part number 725-30009-48 in this catalog for an example.
https://www.eastmfg.com/uploads/2/3/8/5/23858093/east_tool_boxes.pdf

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-