Generators charge better with more RPM's.
Since running these old engines at high RPM's isn't such a good thing, is there any way of changing the pulley on your 12V gen to increase it's RPM's without having to rev the motor?
I had a nice 12V gen on my old '58 Dodge, and it kept the battery topped off even at idle, when running extra lights, stereo and CB.
It worked much better than the one that came in my truck.