Starter clicks using the key, engine starts by jumpering the relay

That makes sense now, thank you.

It's funny you mention that small ground wire, because ever since I bought the car, I noticed that small ground was cut right at the post, so that ground is nonexistent. I'll fix that next time I'm under the hood.
Not trying to lecture, but it didn't occur that was important? We all should keep in mind, that, whether Mopar, GM, or Ford, those guys did NOT SPEND A DIME (or a nickel!!!) on something that was NOT required to make a vehicle operate.

I recommend a better ground. Your engine, where LA or B/RB, has at least one bolt hole on the end of the head. Get a short bolt, make sure it does not bottom, and bolt a Ford style "starter cable" AKA an eyelet to eyelet cable--from the head to the firewall. Sometimes the master cylinder studs work for that. Otherwise, find a spot on firewall, drill through, scrape clean, and bolt the cable to the firewall.

Also some these girls had 1 or 2 small grounding flex cables from the block to the firewall.