I saw you over at the other forum. My screen name over there is Kern Dog.
I have read every post here. I'm surprised that nobody has asked you about the ignition switch in the steering column.
These usually do not fail immediately, they fail intermittently over time. I'ver seen the molded connectors get brittle and crack, allowing a loose connection. I recall reading that members have mentioned to check all of the connections. This means every place where a wire is connected to another wire by way of a clip, a plug or even some electrical tape.
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I do this: I'll turn the key to the RUN position, then get out and trip the relay with a screwdriver. If it fires up and runs, I usually suspect a problem with the wiring under the dash somewhere. If the car spis over but does not start, do THIS: Remove the #1 spark plug wire. Insert a spare spark plug in it, then trip the relay and let the engine spin over. If the spark plug is getting spark, you then have either an ignition timing issue or a fuel issue.
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You mentioned that the car has NOT been able to run since the "upgrades". There have been many times that I have done the same. You bump or move something while working on the car and don't even know it.
The seat belt interlock system may still be a factor here. In the 1974 model Dusters I have dealt with, there is a plastic "brain" for the system under the dash. It is a small computer board housed in a light blue connector about 1/2" X 4" long. It has about 10 wires running through it, maybe more. Some wires (Yellow) run to that reset relay you posted. The others run to the seats, the dash warning light, a POS and a ground....
The TWO yellow wires that lead to the starter relay are NOT there on the older cars. Pre 1973 cars only had 1 yellow wire to the relay. The OTHER yellow wire is for the Seat Belt system.
