Charging issue '70 Swinger

It appears that you have two separate failures, both of which are contributing

No1 problem is the voltage drop in the harness. You have GOT to find that, and as I said, your no 1 suspect is damage in the bulkhead connector. Pull it apart, inspect, clean, replace connections, or spice in bypass wires and just thread them through the holes

Another thing you can do so far as the ignition/ regulator drop is to install a relay out on the firewall to operate the ignition / regulator. You can buy "Bosch style" relays with a mounting hole, run a fuse from the starter relay stud to the relay, and use the relay to "fire" the ignition and regulator.

Second, you DO have a lot of drop on the charging lead. For NOW I would consider doing a temporary (or permanent) bypass of that circuit. Read the MAD article:

http://www.madelectrical.com/electricaltech/amp-gauges.shtml

from here:

http://www.madelectrical.com/electrical-tech.shtml

and basically, run a nice big, say, no8 wire from your alternator stud directly over to the stud on the starter relay

But you STILL need to examine the bulkhead connector, clean and fix it.

I'd bet these two things will go along way towards getting you down the road.