The alternator has to be turning at 1200 rpm to start charging. No idea what that would be on your engine rpm (depends on pulley diameters as you know).
Check out the Genesee Products link below - they have been a GREAT help and offer wiring kits, electronic ignition, etc. They also have some really good explanation links.
They helped me determine that my "one wire" alternator that came on my B (that would charge but wouldn't drop below 7 amps at any time) had a problem and gave me instructions on how to fix it (and sold me the parts). Great folks to deal with.