1965 Dart Charging System Question

0.6 V Does seem to be a somewhat big drop between the battery and the ignition with nothing else on. Could be battery feed or ground.

If you want to narrow it down..
With key in run, measuring at the alternator output will show the voltage at the welded splice (since there's no current in that leg)
upload_2019-12-7_19-33-0.png
If there is drop then measure between the battery positive and each of the next junctions.
Also check battery negative post to engine and body (grounds).