From memory, I think it was '56 when Ford switched from 6V pos to neg grnd.
If I'm remembering correctly Ford, Mercury, Chrysler products, Nash, & Studebaker all had 6 volt positive ground systems through the 1955 model year. I can't remember about Packard & Willis. GM products had negative grounds much earlier and went to 12 volts in the first half of the 50s. Chevy & Pontiac were the last to change to 12 volts (1955?). In 1956 everybody had 12 volt negative ground that I can remember.