Sorry, couldn' resist... All in fun.
The front "half" of the rotating mass is "internally balanced" (no changes outside the main bulkheads). There IS a slight imbalance in the rear, about 6"-oz. The flexplate has a series of holes all the way around. To one "side', there's an "extra" set of 4 holes, about 1" in diameter, not "reflected" on the opposite side. That's the "change" to bring it "in".
A balance shop can duplicate this imbalance on your flywheel without having to "rebalance" the engine. If a "stock" flywheel and unmodified, it's probably okay "as is", but I would have it checked to be certain.
This applies to all the later Pontiac V8s except the 265/301.