Help stranded no power!
Are you checking "inside the cabin" with a load turned on? EG: Let's say the problem is right there in the bulkhead connector. If "everything" is shut off and you show 12V, the connector might be "making" good enough to operate the meter. Do your testing with the key in ACC and the heater or other ACC, radio turned on. This places a load on the connectors
Did you check at the ignition switch connector where it enters the column?
Does ANYTHING "that is live" work? Stoplights? Headlights?
If "nothing live" works then this almost HAS to be.............
fuse link
bulkhead connector the red wire feed
ammeter connections or the ammeter itself
bad "welded splice" in the black ammeter wire
HOW TO GET HOME
If you have no brake lights, but can get someone to follow you and act as a guard so to speak, just hot wire direct from starter relay terminal to either ignition (MSD) or the "key" side of the ballast so you don't burn up the coil. Jumper the relay and it should run.
NOTICE!!! If you look at the 73 service manual, page 8-154, which shows the alternator / starter relay / ammeter wiring, it is CONFUSING AS 'ELL
Look up in the far top left corner at the alternator. It says "with 60A alternator" and "without 60 A alternator" This diagram shows BOTH wiring schemes, and shows the grommet in the firewall for ammeter wiring. THIS IS ONLY USED on the optional 60 alternator. All others use the same old "bulkhead connector" feed through.