an electrical nightmare , please help , 1972 dart

I'm sorry I cannot digest all that inonegreatbiglongsentenceruntogether

1...Pick one major sympton and troubleshoot THAT
2...You obviously have a voltage/ charging problem that seems to be changing. SUSPECT wiring problems not components.
3...TERMINALS, CONNECTORS, SWITCHES are almost always the problem
4....Establish "the path" of a circuit (with the shop manual) and FOLLOW that path. If you cannot get to "one point" in the path, then jump past that temporarily and come back to it if symptoms/ or testing points back there

For example, "getting to" the ammeter or the welded splice is fairly difficult. But you CAN "get up in there" and probe the bulkhead connector and where does the welded splice go? To the ignition switch and to the fuse panel "hot buss." You can easily access the fuse panel. If you thing there is a drop in the ammeter/ welded splice area, test at the fuse "hot buss" and backtrack from there

FIELD REGULATOR WIRING. This IS a problem and I have written endless posts about this. This voltage drop problem is in the VR power supply path, IE GROUND (rare) and the harness VOLTAGE DROP

The problem, the voltage drop CAN CHANGE AS you run/ drive the vehicle. Therefore the charging output can change.