Mike, you need to rethink your wiring for the 3 prong sender. It will work fine for the choke heater, but the oil idiot light won't work. The oil light needs a ground when the oil pressure is low or not there. The original sender provided this ground. With the sender wired "hot from the ignition switch", you will have two conditions. 14 V on both sides of the oil idiot light with the switch closed. 14 V on one side of the light when the switch is open and no ground.
Either way, no light.
If you run the common connector on the sender to ground, 14 V to the choke heater and the choke heater ground wire to the sender, both circuits will be complete to ground and isolated.
