The issue is, that restriction the thermostat provides allows for pressure buildup in the system to further protect from boiling the coolant in the event that you do start to run hot. Maybe try a balanced sleeve thermostat (the "hi-flow" ones you're seeing).
Also, what's your tune? Initial timing, are you using vacuum advance and if so, manifold or port vacuum to the distributor? Airflow is indeed the most important factor for idle temps, but ignition timing plays a big role, too.
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1979 Trans Am W72 400/4-Speed WS6 - Starlight Black Hardtop
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