The stock motor is "Externally balanced" it was cheaper for GM to add the counterweighting to the flywheel and balancer. The counterwieghts on the crank were already about max size for the block design.

This same motor can be "internally balanced" by using neutral flywheel and balancer. Then add "heavy metal" to the counterweights. Expensive. But the aded weight is more centrally located and reportedly is easier in the main bearings.