What carb fits my GMC?

:banghead:I am looking for an original carb match for my GMC T16B one and a half ton farm truck. I have contacted many carb companies, GMC, collectors, other forums, etc. I do not know for sure what year the truck is- I strongly believe it is a 1935, '36, or '37. I have been told that it took a Zenith one barrel. The motor is an inline six cyl with the numbers 408003 by the fuel pump and 408009 by the distributor. I have been told that the motor back then was made by Oldsmobile for GM. I'm not sure if they were merged yet or not. I'm not even sure if Chevy and GMC was merged then or not- their trucks differed in at least grilles and bodies. I have tried searching for books that decode motor casting numbers so I could at least nail down the year of the motor and then back track to specifications for the carburetor. No luck yet. Does anyone know the title of such a book? Thanks.