Ford switched to 12V in 1956, but it was a positive ground system. All auto manufacturers standardized at 12V negative ground in 57, but a few oddball big trucks like Freightliner kept the positive ground system through the mid-1960's. Nash and Packard, and possibly Studebaker also ran 12V positive ground systems for a short time prior to 1957. 6 volt MOPAR cars and trucks were positive ground until 55 or so.
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!