This is a question for the guy you purchased the kit from, it could be balanced either way. CHP does in house balancing also for an extra cost. So ask your friend if when he ordered the kit if he had it balanced before it was sent out, or if he took it someplace to have it done. Obviously if he provided you with a 50oz balancer and fly wheel that is what he was using correct? CHP will do either 50oz or 28oz depending on what you want. They recommend having a 28oz imbalance, especially with and after market crank. This is supposed to save the integrity of the crank at high rpm.
If your counter weights are full of holes then it is most likely a 50oz imbalance.
As for the mechanic , he is wrong. Not all 347 engines are 28oz. Most places do it that way , and recommend 28oz to keep the crank from aerating the oil so much and to maintain crank integrity, but they can be and are done either way.