I'm an air cooled VW drag racer and we have been through the thrown belt thing many times. Basically as said above, you have to make absolutely sure that the belt alignment is spot on. By using a long piece of aluminum angle against the pulley you can measure how far the belt is away from the gauge at near the alternater and it has to be exactly the same at the other end in both directions. A little shimming, or bending might be in order to align everything correctly to keep it on at high RPM.

By trial and error we have settled on smooth faced DAYCO "TOP COG" belts on our race car and have never had a problem at 8000 RPM. They do not streach. We currently run just a mini alternator off the crank pulley but have also used this brand with our old air cooled fan without any problems at 7500 RPM. We buy our belts from FAT Performance in Orange, CA.