GM switched to 12v several years before they went to alternators. You should be able to find a 12v generator that will bolt on in place of the 6v unit. 55 to 57 Chevy without power steering will fit and probably other makes and years up till the change to altenators. The oval metal tag on them will say if it's 12v or 6v. Generators don't charge at low speeds so an alt. is a better choice unless you want it to look stock.
I used a 6v starter on an old Ford with 12 volts for over 10 years without a problem.
The voltage converters Dougie mentioned will only be needed for the heater motor and the original AM radio if you keep it stock.


Originally posted by suedeplymouth
Whats wrong with a 6volt system? Both my daily driven 53 plym and my almost never driven 42 ford jeep still have the original 6volt system and start just fine. The only drawbacks are Id like to have a radio, cant find a 6 volter in fm and the headlights are pretty dim.
You answered your own question suedeplymouth. No FM radio, dim headlights. And on short trips to the store and back you draw more from the battery than you put back.