Coil based systems can run on 12 volts, but the points contact will wear quicker. Points system do use full 12 volts on the start circuit, them drop to 8 or 9 in run circuit byway of the resistor wire. GM used a 12 gauge wire to feed the voltage to HEI, so they wanted to make sure it had full battery voltage.
Aftermarket HEI or conversions differ from vender to vender.
