39 is a new body style, but shares the guages with the earlier trucks. 39 and 40 have the same body, basically, but have different beds and the 40s share the guages with the 41-46.
What this means is that although some parts will interchange over all the years, 37 and 38 share a lot more body parts than 38 and 39. The Mechanicals should pretty much be identical between 37 and early 40s trucks as 37 was the year the 216 overhead valve 6 cylinder was introduced.
GMC came out with their own overhead valve 6 in 1939 which was of larger displacement (228) and better design. Prior to 1939 GMC was using either oldsmobile or pontiac flathead sixes in their trucks.
I guess what I'm trying to tell you is that a lot of the prewar stuff will interchange, a lot won't, but your best knowledge base will come from 1936ish through 1946ish owners
John
FYI I have a 1940 AC 152 (3/4 ton long bed)
