READ this excellent article:
http://www.madelectrical.com/electricaltech/amp-gauges.shtml
Even if you don't do this mod, the article explains fully how these old girls work, and the problems that arise in the main primary power system. This diagram from the above page
is a simplified look at how your system works. NOTICE that the main power comes through connections in the BULKEAD CONNECTOR. This thing is a disaster waiting to happen in these old girls. There are several connectors in there, not the least of which is the battery and alternator feeds, which can give trouble.