39 and 40 have the same wheelbase, 41-46 is a little bit longer. Somehow they stretched the cab about 2 inches in 1941... makes for longer running boards. The beds changed from 39 to 40 as well as the dash. If your 39 has one rectangular speedo cluster... you would actually have a 1940 model year. I believe the guages are identicle for the 37-39 model years.
