James,
Check your chassis with the factory photo of an Cab 11 or 12 FAT chassis below:
(Source:
http://www.oldcmp.net/factcgt.html)
Note it does not have overload springs on the rear axle.
A real giveaway should be the cab. The Cab 11 and 12 Field Artillery Tractors had an integral body, quite different from the G.S. truck cab. If you post some pictures, we could possibly help a little more.
Regards,
Hanno