6 volt coils don't need a ballast resistor.In reality,there's no difference between a 6 volt coil and 12 volt coil requiring a resistor for a points ignition.Both will have an ohm reading between the two primary terminals of 1.2-1.5 ohms.I have checked this out many times on GM coils.
And again,a clean spark plug needs only about 3000-5000 volts to fire in an engine with somewhat rich carburation and low compression(like old truck engines)The original type caps and typical 7mm plug wires don't have the dielectric properties to cope with 40,000 volts.High voltage ignitions are needed in emissions engines running lean mixtures and modified high compression engines.The old 60's Muscle cars manged just fine in their day on stock coils and wimpy 7 mm carbon wires.
