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I have a 1958 Chevy C60 Viking and I get along great with my generator. I don't drive it much at night but when I turn my headlights on bright and speed the engine up a little the amps gauge still shows it is showing enough charge to be to the plus side.
It is normal to have to rev the engine some above idle for the generator to charge. As far as installing a smaller pulley on the generator, you should be able to rev the engine up farther to see how a smaller pulley would work better. I doubt if it would help that much.
There is also the matter of the voltage regulator, which may or may not be up to par.
I'm not trying to steer you away from an alternator if that is the best way to go. Just sayin.
