I would go with the 12 volts to start it. The fresh rebuild will loosen up quick enough once its started. I had the same problem with a straight 8 Pontiac. New rings and fresh cylinders, the 6 volts was to slow to get it turning. After a break in run. The 6 volt had no problems. It may be best to replace the battery and use the factory cables. Just shut off all lights and radio. The extra voltage will help! Joe
