In case it may help, I did some web surfing and came across some pictures of an out-of-stock 1940 Chevrolet Truck Data book HERE. They nicely enough have left the pictures of some of the book contents posted and the page showing the truck and bed general dimensions is there for all to see. grin I took a screenshot of the pic and have attached it below so you can see the 1940 1/2 ton bed was listed with a 78" long (inside) x 45-1/2" wide (inside) dimensions. That makes it 1" longer than the '37-'39 beds and the same as the '41-'46 beds.

Just trying to fill in the information gap in case it may be helpful. smile
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~ Dan
1951 Chevy 3 window 3100
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"My Grandpa Carl's Truck and How it Became Mine"
1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver)
US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)