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

Well with that info, it becomes somewhat more clear WHY you are having such problems. If indeed someone has morphed a 70 and 74 harness, that is even more trouble.

Frankly, I see no way out at this point other than either repop replacement harness or starting over with a universal harness

Thing is, either you or we, not even knowing what harness is there, and all chopped up, you can't really go by a diagram