I want to use my original 6 volt coil in my 12 volt system; what kind of resistor do I need to use that will protect the coil?

ron

When properly wired a 12V car coil recieves 12 V when the key is in the cranking position and about 7V in the run position. If your car isn't wired in this manner (as the originals were) and you add in a resistor the coil will be getting the 7V at all times. I think that you will find that the 6V coil will preform OK with the resistor. You can use a 1955-1959 GM car resistor. They are quite available.