Bad Alt Regulator?

Some surging/dimming of your lights during starter operation sounds normal to me. I do not know if the original regulator was mechanical on the '69s but they were on the '67s. Obviously no idea if your replacement is solid state or not either but the original mechanical style is easy to identify by looking at the backside. Mechanical ones had wire coils that would heat up at certain loads and flip open to break the circuit.

If your lights are doing that under normal driving it could be several things. Have you checked the alternator itself? Some alternators are internally regulated, I could see the alternator and the external regulator arguing with each other. Which lights are you seeing this on, headlights, dashboard or all of them? Again I've never dissected a '69 but my years also had regulators behind the gauge pods, I'd look at that if it is your dashboard that is exhibiting the problem and not the headlights. Another common part to fail after so many years is the headlight switch itself. The old designs were engineered so that the switch carried a full load too.

Sorry I cannot be of more help.