With the dimensions and new photos provided it appears that the bed sheetmetal is likely from a 37-38 Chevy. 39 and 40 beds had the "Chevrolet" script in the tailgate and the 40-46 beds were about 48-1/2" wide inside width and 78" length. I don't know about the fenders. Mar-K and other vendors have the correct reproduction 46 bed parts or assemblies available. Just be aware if you purchase a bed part such as a tailgate or front panel for a 46 it won't fit properly on your earlier bed. If you keep your bed and choose to repair it be aware that some of the 37-8 parts may have been modified or changed to fit the 46 frame.

Mark