John,
The 1937-'39 beds were 77" long and 45 3/8" wide.
In 1940 they went to a width of 48 1/2".
In 1947 and up they went to a width of 50".
In 1947 there was a change in that the front bed sill bolted to the front bed panel.
Most of what little I actually know about this topic has been found in the booklet from Jim Carter, "Bed Restoration Manual for Early Chevrolet and GMC Pickup Trucks". I have found that a lot of "Instructions" are less than clear.
