I typed this into Google:
Cowl tag on 1959 GMC?
http://www.1955gmc.com/1955-59gmcguide.html
Post #72 in this thread:
On chevy trucks there is no firewall cowl tag, the vin plate is inside driverside door and should match vin on frame by steering box and also marked below cab on top of frame.
I'm assuming the same for GMC.
CAB AND BODY IDENTIFICATION
(Blank Plates are available-Part #2267999-Model 300 & up)
The "GMC Truck Service Parts Identification" plate mounted on inside of glove compartment door, provides identification of the major units of the vehicle. This identifying data also is shown in the parts listing. Therefore, reference to this plate before ordering parts, will insure correctness of the parts ordered. Cab and Cowl Models used on each chassis model can be determined by reference to following pages. Body models are shown in group 23.0005.