Need help with overflow plumbing

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If I'm clear, you have an external overflow with 2 holes.

You're just about already a classic Herbie; just remove the Stockman, cut off the lower drain line from the Tee, adjust the valve so you don't flow anything into the emergency. Since the emergency is normally dry you don't need anything to silence it.

Calculator at bottom of this link (Beananimal) should give you idea of max flow on a 1"