How many cells in the battery (how many caps/opening in the top, if there are any)?
What brand/model battery charger do you have? Are you sure it is indicating 12V rather than 12 amps?
Chevrolet trucks had 6v negative ground through mid-1955; GMC trucks had 6v positive ground (in 6 cylinder trucks through 1955).
I think that 6v bulbs would not have worked for a long time in a 12v electrical system.
