Are you referring to an original GMC 1956 V-8 (maybe a 316 or 324)?
I think that all 12v GMC electrical system were negative ground.
Through 1955, all GMC 6V 6-cylinder electrical systems were positive ground. In 1955, GMC 8-cylinder electrical systems were 12v negative ground (except V-8s in school buses were still 6v).
Chevrolet changed to 12v for all engines in the 1955-2nd series trucks.
