bosshog,
when the change was made to 12 volts ( 1955 ) the number of teeth on the flywheel ring gear & starter drive gear changed. even though the 12 & 6 volt starters look alike the drive gears won't interchange. early starter-early flywheel. late starter-late flywheel. if you put the starter that came with the gmc on it you should be ok.
robert
