Voltage drop with headlights on

[craiginaustin]

I have an electrical gremlin that I'd love some help narrowing down. When the headlights in my 64 are on, the reading on my voltmeter drops down to around 11 volts (sometimes less). The gas gauge also reads about half of what it reads with the headlights off. The same problem, but to a smaller extent, happens when the indicators are on (you can see the voltage / gas gauge drop in sync with the blinking). The voltmeter is wired straight to a spare IGN 12V slot on the in-cab fuse panel.

I plan on checking the alternator output voltage to make sure it's not toast. How can I tell if it's an internally or externally regulated alternator? If externally regulated, how would I find and test the regulator?

Could it also be a bad ground somewhere? The truck seems to be lacking grounding wires / straps in general -- I added one from the engine block to the frame (with a block to battery ground already in place), but I don't see any cab grounds at all. I believe there should be one from the firewall to the passenger-side head? Where would I check for the presence or lack of grounding to the in-cab electrical systems?

Thanks in advance for any advice. I'm relatively electrically savvy, but automotive electrics are pretty new to me.