Hi John,
Welcome to this web site! Sounds like you have a really rare bird!
I don't know much about the GMC trucks, but I can offer some insight to how some of the Chevy trucks were built.
The chassis in my '28 Canopy Express was made by Chevrolet, however the body was made by the Hercules Body Company. In the 1920's, all the Canopy Express bodies put on Chevy chassis' were made by subcontractors, such as Hercules, Martin Parry, and Springfield.
Your truck might have a similar story. Post some pics! We all would love to see them! If you upload the pics to be online somewhere, like photobucket, you can include a reference to the pics in one of your posts here.
Cheers, Dean