I hope page 22 in this link helps a little bit? GM Heritage Center 1941

If the tailgate you got is 58 inches wide that's not off a GM half-ton or 3/4 ton of this era. I can measure my top pipe diameter and box width when I get home. I can't give you a reliable measurement on box side length though, someone else will have to chime in with that. If your sides are 79.5" long, they sound about right to give you that 78" inside length shown in the manual I linked to.

Can you post photos of what you have? That might also help.

1946 GMC Pickup - S-10 Frame, 455 Buick, TH400, original patina.

My 46 GMC on Photobucket