Charging Problem

First I suck with electrical problems. Sorry in advance that this post is kind of all over the place, much frustration right now. Here's what started happening last night. my car 66 valiant with a 318. First I found out my battery was dead just picked up a new one and have the same problem. My volt gauge appears to be charging at about 11.5 volts. I recently had the fuel gauge rebuilt and it was working fine until now. While driving on flat road it dropped to empty and then went back up (did this twice) now it is stuck on empty again. Alternator is new. Just took a test drive and volt gauge is holding steady at about 11 or 11.5. No ammeter hooked up. No electrical was running. When I got it back in the garage tried turning the headlights on and the gauge dropped way down. No change with heater or radio. Could voltage regulator be bad. Or is it more likely a short possibly from the fuel gauge. I do have a volt meter but don't have a clue how to check things with it. Any pics of how to check would be great. But any advice would also be helpful. This sucks it's a beautiful day to cruise. Thanks in advance.