Time to properly rewire the whole car.
make sure you bypass the ammeter gauge and the bulkhead connector since they are the weakest links in the system.
No matter if it is a 700.00 car or 70K car I always run an extra wire with a fusible link between the alternator and starter relay to take the load off the ammeter gauge.
Any corrosion at the bulkhead connector must be cleaned--or if really bad replace it.
Always use diaelectric grease.
With a multi-meter check continuity on each circuit.
I bet if you open up the harness from the bulkhead to the column you will see wires melted together.
Not a uncommon issue with 40+ year old wires cannot handle the resistance anymore.
Especially if there are any aftermarket add-ons putting a load on the electrical system. (cooling fans,stereo,lights,hotter ignition system,etc).
Hope this helps and good luck.