Charging voltage is a common issue with old cars. I've found the main culprit is the firewall connector. First measure the voltage at the alternator terminal with the engine running. Use the engine for ground for this. You should be measuring 13.8 - 14.2V on a properly running electrical system. Then measure at the battery + terminal using the engine as a ground. You will notice a drop. Turning on various items will make it worse. Check the ground lead at the back of the engine first. It needs to be in good condition and tight connections. After that the wire from the alternator to the firewall connection needs to be in good condition along with the terminal in the firewall connector. Same goes for the connection from the battery to the firewall connector. If you disconnect the firewall connector and look at these terminals I bet you will see they are corroded or worse, partially burned away. I won't go into the physics of this. You can replace the terminal and solve the problem for a while. They can be found on ebay or at delcity.net under Packard Type 56 series. Good idea to order several of these for future use. It is a pain to crawl under the dash and find the mating end. It's also difficult to remove that end with the connector block located where it is.
What I did was replace the wire from the alternator with a larger one (8 Ga) and drill a hole in the firewall for the wire and a grommet. Then splice it under the dash with a butt splice. Soldering this with a mini torch (found on Amazon) makes it reliable. Did the same with the battery connection. Insulate the connection to prevent fires. You want to use high quality wire here, a lot of the cheap stuff is really copper coated aluminum wire, avoid this. Get the "oxygen free" copper wire. You only need maybe 15 feet. Measure to be sure. If you plan on adding a bigger alternator eventually and other high current items (EFI, electric fans, fuel pump, stereo, etc) this wire should be even bigger (4 Ga or bigger). A fuse needs to be in the path from the battery to the firewall for safety purposes. Don't go above 60A without serious evaluation of your entire electrical system. The original fusible link was meant to self destruct if this was exceeded.
Later models of cbodies avoid a lot of this by not running the alternator -> battery current through the firewall. It is a direct connection with small wires connected at calibrated points going to the ammeter gauge. It really isn't an ammeter anymore, it's a voltmeter measuring the drop between these calibrated points. Partially solving the high current connections at the firewall.
