The Clark wiring of the day was such that the Delco starter had a terminal on the solnoid that was energized only during cranking (not by the igntion switch). The wiring to this terminal was connected directly to the positive side of the coil, bypassing the ballast resistor.

I have had to dupicate this type of wiring schematic (with a relay) in numerous applications when the power to the igntion coil drops off due to high current load to the starter.

Changing to a external ballast resistor coil, adding the basllast resistor & relay with the related wiring has saved many a truck from needing a replacement wiring harness