Please help!! 1971 340 duster having major ignition wiring headaches.

When the engine is running. Power is supplied from the run circuit.
Electric current is like water flowing though a pipe.
It flows from the alternator or battery to junctions where it branches off to circuits that need it.

Engine running, current flow looks like this.
1702222255559.png

When the ECU closes the connection, power flows through the ballast resistor and coil. The resistor reduces the current flow and a voltage drop across the resistor can be measured.
The coil negative wire is connected to ground every time the magnetic pickup signals the rotor has moved to the next spark plug.
The Chrysler 4 pin ECU is powered through the light blue wire. It draws power whenever the key is in Run (or start).

So the resistor drops the voltage in proportion to current flow.
1702223076037.png
Resistance varies with temperature, current flow varies with coil and rpm so I can't give you an exact number. An osciliscope is more useful than a meter for checking coil primary voltage.