Bad idea on an injected car, however if your game, then installing a break in the wires which supply the field coil (?) will cut power to the alternator.
If you have underdrive pulleys, then you've probably did more than enough to compensate for the draw created by the alternator.
Besides last time I read, the alternator only sapped about 2-3 hp under a normal load. Avoid running the AC, your mucho stereo system and the head lights/flashers while going down the track and you'll be okay!
