[B L & sons]

Dave
I would not think the problem is with the battery or your underdrive pullies.The underdrive pullies would turn the alternator slower than stock.(I believe thats right)Your alt should put out 13 volt range.If you have a NAPA are auto parts store near by thay should be able to check them without taking anything out of your car.I would think its your regulator.
Hope this helps
Bill