[JSeery]

What is often called a 6v coil will work fine on 12v, positive or negative ground. As you posted, it is the current in the circuit that needs to be controlled in the 3 to 4 amp range. With a "6v" (really one in the 1.5 ohm range) you need a ballast resistor (also in the 1.5 ohm range) for use with 12v. So, you are looking for ~3.0 ohms of resistance for a 12v ignition circuit. Or, you can use what is referred to as a 12v coil, really a 3.0 ohm resistance coil, as it provides the 3.0 ohms of resistance without a ballast resistor. The polarity of the coil needs to match the polarity of the rest of the system, so a negative ground system has the negative terminal of the coil going to the distributor points & capacitor and the positive terminal of the coil connected to the ignition switch.