I had a 350 in a Camaro that would throw a belt almost every time I drove it. New belts didn't help. It turned out to be a defective alternator pulley. It lined up with the crank and water pump but had a section where it was slightly wider at one point. This was like a flat spotted tire where at that same point every revolution, the belt tension would change just enough to upset the contact and the belt would bounce out of position.