39 and 40 share most body parts and mechanicals... a fer differences would be in the dash with the guages and knobs. other than that a 39 and 40 1.5 ton truck should be identical.

I have a 40 gmc 3/4 ton, and there are several other members here who have pre-war trucks here. As most of the trucks of the era share drivelines and mechanicals you can learn a lot from members with trucks in the year range of 37-46. Not all parts interchange, but to give you an example... 37-39 share the same dash and guages. 39- 46 share the same doors and windshields. 39 and 40 have nearly identical cabs, while the 41-46 trucks had a cab that grew 2 inches in length.