Ampmeter flickering

Yup. This could be one of several problems. Could be a bad connection in / at the ammeter or bulkhead, or a problem elsewhere

Might be a bad regulator

Might be bad brushes in the alternator

Might be bad grounding between alternator (engine block), body, and battery

Or a poor connection "voltage drop" in the engine harness supplying ignition power to the voltage regulator.

What are you using for a regulator? Is this the old 69 / earlier mechanical regulator?

What year / model we workin' on here?