Start and Run are differnet routes.
Voltage drops when current is flowing through resistance. When there's no flow, the resistance goes away. The poor connection of course hasn't gone away, but its not causing any resistance because there's no flow. If your seening voltage drop through anything other than a resistor then two things are happening.
1 Current is flowing so so there's a connection from positive to ground. That shouldn't happen at the coil when the key is off.
2. There's high resistance in the circuit between the two voltages. Find where it drops and you've found the resistance.
You made one electric change, the electric choke. I'd suspect that first.
Some Chilton's are sometimes useful but not for stuff like this.
Bishko sells reprints.
There's also thread on FABO with a title something like Free Manuals.
I know there's a '73, I don't know about a '74.
I don't know if disconnecting the interlock wires was the correct move. Do a search on that.
I would not leap to connecting the coil directly to the battery. That's a bit risky and desperate move IMHO.
It could be as simple as one of the terminals backed out of its connector.
PS. When you see a voltage drop and the power is coming from the battery, glance at the ammeter. It shows approximately how much current is flowing out of the battery (except what goes to the starter). It should be pretty small current 2 to 5 amps for starting.