If you over carburete then the velocity of air through the venturiis slows down, thus, through Bernouille's principal, you will lose the vacuum signal that pulls fuel into the air stream. Shutting off the secondaries and not losing performance should tell you that the engine has a big enough carb with half of that 390 carb. Stay with vacuum secondaries. The 2ndaries will open only when engine airflow demand requires it.
