The correct Ford ignition resistor has an element made of nichrome wire, such as in old toasters. As the voltage in the system increases, the winding heats up, increasing resistance to keep the voltage to the coil in the 3-3.5 range. Ordinary voltage droppers don't do this, usually resulting in burned points. If you convert to 12 volts, have the coil rewound so you can eliminate the resistor on them firewall.