Whit, There is nothing wrong with a 6 volt system on a old Cadillac as long as the cables are heavy & the battery is high cranking amps. The reason most people convert to 12 volts is they can add A/C, stereo, electric fan, & charge there cell phones on a road trip. Also you can find 12 volt parts when you need them. Other then that a 6 volt system works great.
Lee DeVol