headlights are dimmed and turned signals won't blink

You could have a number of troubles bundled together.

First, how's the idle speed? Can you crank it up a little? I like to run mine "a little high" just not high enough to cause annoying creep in gear

Second, suspect the alternator. You might have one or two open diodes, which will turn, say, a 50a alternator into about a 30A alternator, with much less efficiency, and it could have other troubles.

Measure your running chargiing voltage, at the battery, while running the engine at an RPM to simulate "low to medium cruise."

How big/ heavy/ good is the battery? If the alternator dies down at low idle, it surely doesn't help if you have a low end, old battery that's on it's last.

Also on the list, which pops up here every day, is the condition of the bulkhead connector on these old girls. This is the huge connector on the firewall where all wiring feeds through from the engine bay to the interior. These deteriorate, the connections become hot, burned, corroded, etc.

And, there could very well be other issues--rusted, corroded, loose fuse box terminals, connectors at various points including the ammeter, the ignition switch, the switch itself, etc, etc.

Your ammeter action is "normal" giving your situation. What is happening, the alternator is "not keeping up" at a low idle, the battery is providing power and discharging, and the system voltage is going very low. Then, when you rev the engine, it charges for a short time until it once again gets the battery normalised.

Now PLEASE don't go out and start throwing money and parts at the problem.

Follow along with other comments, make tests, and make the most logical choice.

You might not find it in a day or even a week, but, hey, I'll bet it's been going on already, right?

By the way, those of us old enough to remember riding in cars/ trucks made in the '50's think this was normal operation. Just wait until you try out "vacuum wipers."