"12V" GM coils are actually made with a primary (low voltage) winding that works right at around 9-9.5 volts. That's approximately the voltage a 12V battery has available during cranking, due to the load the starter puts on the electrical system. The ballast resistor is there to drop the voltage available to the coil low enough to prevent point damage when the charging system is operating against a fully charged battery- - - - -around 14.5 volts. During starting, the resistor is bypassed to allow for a hotter spark with the low battery voltage that's available. GM coils from about 1955 to the early 1970's (before HEI) should be interchangeable, regardless of the vehicle they happen to be listed to fit. Ford and MOPAR "12V" coils usually don't have the right primary winding resistance to work well in GM applications.
Jerry
