How Much Work is this: Converter

That's right.
But first; what is the idle in-gear speed, without the headlights.....and with.
There is a lower limit to what the charging system can do, especially the non- isolated field type,as originally fitted to your car.
The isolated field didn't come onto the scene until 1970 (IIRC). If you still have that older system, it can easily be converted. It will charge at a higher output at the same low idle rpm.
Another option is to re-configure the pulley sizes for more alternator rpm at that same low idle-rpm. But this has the effect that at the other end of the scale, namely at redline, the alternator rpm may be high enough that the belt keeps jumping off, which is a bad thing, cuz then the engine quickly overheats.

And finally. the most common problem is the bulkhead connection. A high resistance connection here converts the electrical power to heat, instead of feeding the headlights.
And the second most common problem is the headlight switch itself. Again, a high resistance connection here, converts the incoming electrical power to heat, leaving little left over for the lights.
The regulator will sense the power loss as a heavy current draw, and kick the alternator into overdrive trying to keep up. Of course the engine senses the load and the idle speed drops because of it.