Starter stays engaged ONLY in park???

You probably need to find some local help. I don't mean a shop necessarily. Isn't there another A bodies member in your area? Feel free to give me a buzz

Here are the main points I "imagine..........." in no special order

1...Possible wiring hacks by others. Especially since the old seat belt interlock seems to not "follow" someone may have hacked that. Who knows maybe they only twisted the wires and taped them into the harness, and now they are not making connection
2...Problems with the bulkhead connector. This is age old on these cars. Only way to deal with it is learn to unclip the connector sections and pull them apart and inspect the terminals. You CAN buy these terminals.
3...Failure in the ignition switch. It sounds like "maybe" you need to modify your testing methods. I RARELY use ohm/ continuity tests, because a "poor" but "somewhat" connection will often "pass" a continuity test but fail under operational load. The answer is to test under load. Make voltage tests UNDER load instead of continuity

For example confirm or fix a correct path for the start wire. The path--after eliminating the seat belt interlock is........from the key switch connector, through the bulkhead, and out on the engine side to the start relay

So start at the key. Learn to "jerry" rig if needed--clip your meter/ light into the start wire terminal at the key switch connector, and see if "it's there" in start. If so......

Go out to the engine bay side of the yellow wire and probe that. If it's not there then there's only one place---within the bulkhead connector

NEVER assume a terminal or switch or crimp splice is "good."