Alternator, voltage regulator, or battery?

JMO,
Upgrade the alternator to the dual field 60-66amp and switch to a electronic voltage regulator, the old one is mechanical which makes for poor transition between charging.
Be suspect of the amp gauge in those old a-body's a lot of the time they go bad [ they start flickering/jumping ] if this happens get a stewart warner amp gauge and run the wires from the old gauge to the new.

Even after these things you will still notice a lil head light dimness while stoped at intersections, I would install head light amplifier box.