Starter stays engaged ONLY in park???

No I've told you earlier. Either do voltage checks or resistance checks, I prefer voltage "as long as I can" and then resort to resistance after I've narrowed it down.

Get "the path" in your head just as if you were driving somewhere, you drive so many miles on US95, you turn off at wandering meadows road and go so far, make a Y to the left on red rose lane, and so on.

Same thing with wiring.

"The path" You come from the BATTERY STUD on the starter relay through the fuse link, through the bulkhead connector on the BIG RED ammeter wire, and to the infeed at the key switch CONNECTOR

You go through the ignition switch AND KEEP IN MIND that it is in reality SEVERAL SEPARATE switches. This means if you have a low voltage problem in one circuit, it might be in the switch even though other circuits seem to be fine

So "rig" a way to clip into the various connectors or find a helper. Hook everything up normal. If you want to disable the starter, but still have the relay operate, then just undo "the big square" screw where the solenoid wire hooks. Now when you turn the key, the relay "should" operate and load down the key with that much of the circuit operating (as an example.) Now check voltage at the points you can access EASILY then go back and catch the "hard" ones if they seem to be involved.

So check power at the big red going into the key switch, and read it carefully. Twist key to start, does it drop at all? If it does you might have a problem right there in the bulkhead connector in the BIG RED and the ammeter circuit. If OK, move to the yellow at the key switch does IT drop? If so, might be bad switch or the connector. Check both sides of the connector. Move out to the starter relay. Check the yellow there, and if it drops? You've already checked at the ignition switch, say, "it was OK" what is left? The terminal on the end of the wire, and the bulkhead connector. Move to the bulkhead connector. It drops? Has to be right there in that connector, that is all that is left.

"Just follow the path."

WHAT I SOMETIMES DO

Since the 73/ later factory diagrams are difficult to follow "sometimes" download the aftermarket diagrams from MyMopar. They leave out options but at least they are line drawings.

Another thing you can do is download the 72 shop manual from My Mopar. THAT WILL BE DIFFERENT than yours, but the line drawings in the book might be easier to get into your head.

This is a very simple circuit, now that the interlock is out of the picture. Don't overthink. It might help to draw it out by hand, and take notes what you did for readings