with 12V you should either have an ignition resistor [from a FLAPS, one listed for a 58, any model or engine], or an internally resisted coil [easier], which should be available as Echlin #IC21 or Napa #IC14 [I=internal] - check a wiring diagram for 56 up to see how the resistor and bypass are set up
herethe ignition resistor cuts the voltage to the coil to increase points life, and bypassing it while starter is engaged insures fatter spark under the starter loading, so easier starting
Bill