I would get a new builder. Give kevin at Herr Jugs Racing a call. He won't do you wrong. Great guy to deal with. The broken piston could be caused by excessive piston to cylinder wall clearance, detonation from a lean condition, porting the intakes of the pistons can also caus cracks and breaks. Post up pictures of your spark plugs, domes, and tops of the pistons and we might be able to help you out a little better. Those three things will tell you tons about how the motor was running.