[bigblockv6]

305,351 & 401 V6 engines are internally balanced so flywheels for these engines are zero balance. Swapping compatible flywheels on these engines isn't an issue provided engines still have the factory balance on them. Engine builders do balance the whole rotating assembly together on just about any engines and if they do add or remove any weight on the flywheel then that flywheel will throw the engine out of balance if it's put on another engine. 379, 432 and 478 V6 engines are externally balanced so there is weight added to the flywheel as well as the front balancer. In my 478M conversion I had a custom flywheel, when the engine was balanced a good amount of weight was welded to the flywheel as well as the front pulley.