A quick look at Motor’s Auto Electrical Systems book says that external ballast resistors were used with Prestolite coils of the day. I’ll leave it at that for now. BTW, “Ballast Resistor’ means that the resistance changes as it heats up.
I have a sneaky suspicion that the early OMC battery ignition coils will work for you, if you can fit it in somewhere. EDIT: That would be OMC 0379569, way too expensive to experiment with.
