swapping pistons and balancing

Also, the lighter pistons could be run on the stock crank with the balancer and flexplate. It's not a great situation, but the better of the possible out-of-balance situations one can have. Out of balance means just that, and brings with it all the down sides of that situation. But heavy in the crank is much less of a big deal than heavy on the reciprocating side of the equasion. You could also just spin balance the crank to the lighter bobweight, if you find the average piston weight of the new set. Then it's 1/2 a balance job.