The 2nd.-generation 1955 Chevy trucks made the switch to 12 volts. I'm not sure when GMC swapped over, nor whether they kept the positive ground system for awhile after the switch to 12V. I don't think so. By 1957, all the major auto manufacturers had standardized electrical systems at 12V negative ground at the urging of aftermarket electrical accessory manufacturers. Freightliner big trucks hung onto the 12V + ground system until the mid-1960's.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!