Small block ford are externally balanced. This means that the flywheel and (i think) front balancer are "out of balance" In order for the total reciprocating parts of the engine to be in balance, you must use the correct out of balance flywheel and front balancer. Ford used the fifty oz in the later years. You have to make sure that the front balancer is the correct one, assuming that it is unbalanced. Maybe a ford guy could clarify the front balancer question.