If the coil + voltage drops that far when cranking, the ballast resistor bypass circuit is not active.
Basically what you "had" in the Mopar 4 terminal resistor is TWO separate resistors in one ceramic block
One resistor fed the coil + just like a "points" system, and in fact was very close or the same resistance.
So for that "half" of the resistor, you have:
1---IGN 1 or "ignition run" traditionally dark blue. (I didn't look at a diagram of a 73, might be different color) This "run" voltage is hot ONLY when the key is in the "run" position.
So in "start" you need something "else" to provide start power
There are two OTHER SEPARATE switch contacts inside your ignition "switch" which in reality is SEVERAL switches in the same housing.
2--One is the yellow "start" wire which is working because the engine will crank.
3--The last is known by Ma as "IGN 2" and is the "bypass" circuit. This is a separate switch contact, feeds a traditionally brown wire through the bulkhead/ firewall connector, and goes to the coil + side of the ballast resistor. THIS IS hot ONLY in "start" and is the ONLY source of ignition voltage during "start" or "cranking."
So look at your ballast / harness and see if that brown, coming out of the bulkhead, is hooked up. If so, probe the firewall connector. Wiggle the harness connector vigorously and see if it comes alive. If you cannot "find" power on that wire, get under the column at the ignition switch connector and see if you have power on that wire in "crank." Refer to a proper year/ model diagram to be sure you have the right color.
I have a 74 manual, not certain if it's correct.