Squareback vs round back alternator?

I'm going to throw this out there. Maybe it is just fine and functioning as expected?

If you raise the engine RPM does the fluctuation level out?

Rebuilders are notorious for putting a 70mm dia or 76mm dia pulley on alternators that should have a 63.5mm dia pulley. The seemingly small dia change makes a huge difference in alternator shaft speed and the output at 1000 rpm shaft speed vs 1500 or 2000 rpm shaft speed can be the difference between charging and not charging.

Diameter, diameter, diameter, alternator pulleys

You mentioned with more accessories turned on it levels out.

I agree with Mattax. Get familiar with the workings of the old stuff before you attempt to make changes.

The master tech videos are really good at explaining how things work.

Mymopar.com has a lot of them

Don't get too hung up on the year and the car. Early 60s work the same as late 60s.

Welcome aboard and good luck to you.