Corpo, Go to this website. While this is for Chevy and not GMC it should be close. They have both service and parts manuals and will probably help you with the info you are looking for.

http://chevy.oldcarmanualproject.com/index.htm


1939 GMC AC-102 Long Bed 1/2-Ton
In the Legacy Stovebolt Gallery
More details in his DITY Bay

1940 Chevy WB Series 1.5-Ton
In the Stovebolt Gallery