Suddenly Lost Power

You should also have at least one body ground connecting the engine to the body somewhere. Make sure the point where the cable attaches to the engine is clean. Use sandpaper to brighten things up. Same with the positive cable, make sure it is clean. It sounds like you still have a loose/dirty connection somewhere. Check out the firewall connector, ignition switch connector etc as well. AMP gauge work? A bad AMP gauge will cause all manner of problems like non-charging and total darkness. Consider bypassing that rascal by putting both wires on the same post.

Although I don't think it's a factor in your problem, you might want to take a look at that new alternator you installed. Your 65 originally had a "roundback" alternator with the old mechanical voltage regulator. I am not sure how well the old style regulator works with the later "roundback" or "squareback" alternators. Parts houses NEVER give you the wrong part... A later high output alternator will fry your original AMP gauge ultimately resulting in total darkness.