One way to find out would be to order the paperwork from PHS( pontiac historical services). Just off-hand I would say no it's not possible. The original purchaser could very well have had the dealership he bought the car from swap the motor out but nobody in their right mind would do that. It would cost more doing it that way than if they had just ordered the 400 model in the first place. I would say someone is yanking your chain.
