You can easily "hot wire" the ignition to see if it will run. One way is to pull the blue field wire off the alternator and run battery power TO it. It should be spliced into the ignition. Another place is to figure a way to jumper power into the ballast where it is jumpered together. That is, two of the 4 ballast terminals should be jumpered together.
So far as losing voltage, the functional path is battery.........starter relay "big stud", fuse link, through the bulkhhead on the big red ammeter wire
Through the ammeter, out the blg black ammeter wire, the welded splice, branch offf to the fuse panel "hot" bus and to the igntion switch.
Through the switch, out on "ignition run" usually dark blue, back out through the bulkead, and at some point branch off and feed the ignition system, the VR, some smog equipment if used, and the alternator field
So the main points to check are the big red and big black going through the bulkhead, the ignition switch and it's connector, and the dark blue feeding back out into the engine bay through the bulkhead