I'm not a fan of underdrive pulleys, but some guys run them without problems.
My truck had an u/d crank pulley when I bought it and I went back to the stock pulley - mainly for the a/c compressor performance at idle, but also because I was showing low voltage output at idle.
If your problem is only at idle, then you can get an overdrive pulley for the alternator to compensate. If you're having problems while driving, then it's not the pulley, it's the alternator or voltage regulator.
Recommend you post your future questions to the Gen1 board - you'll get more visibility & responses!
Good luck,