1974 duster starting issue

I would:

1...Rig yourself with a test light and alligator clips, or enlist a friend. Rig the light to ground and to the start wire at the start relay. This will be the yellow of the two wires. Verify that as you turn the key and hold to start, that you get a nice bright light.

If not, check the routing of the yellow wire. If someone bypassed that interlock, maybe they didn't make a good connection at the two yellows. I would unwrap some of the harness over on the fender. Look for suspicious bulk, re-wrapped area, etc.

You can find the bulkhead terminal where the start wire comes into the bay, probe that with your light and check again, with the key in start

If you just cannot get a "solid" light, move into the interior, and acess the ignition switch connector coming off the column. It is a long slender inline connector. Check there with your light to see if you have start voltage. Check the connector for heat damage, loose, corrosion, etc. If you have a light there,, you are still looking at that wire, goes through the bulkhead terminal, to where the reset used to be (bypassed) and onwards to the start relay

2...If you do get a good light in (1) above, now move your light to the other "push on" connector on the start relay. It should NOT light. If not, put the shifter in D, 2, or 1 and twist the key again. NOW it should light, which shows the relay coil is getting power through it, and that the NSS is opening up.

3...If not maybe the relay coil is intermittent or bad. Remove both push on connectors. Ground either terminal with a alligator clip wire, and jumper 12V to the other. Engine should crank. If not, relay should click. If not, replace the relay.

4...Could be intermittent / loose wire from the relay to the solenoid. This is the large about no12 or no19 wire from the "square" screw terminal of the relay, down to the starter. With everything reconnected "normal," and the shifter in park, move your light to the big square screw. Any time the key is in start, it should light. If it does, and the starter does not operate, move your light down to the small solenoid terminal on the starter and recheck.

5..You say the starter operates when jumpered, if this seems "solid" and every time, the starter is good I would say