Vacuum gauge
Put a vacuum gauge (any auto supply) on a manifold vacuum port. A stock 289 should show 15-17 in hg. vacuum. If it's a stock engine and it's alot less, then you do have avacuum leak.
A leak can occur at the head/intake manifold interface or at the intake manifold carb interface. Also a leak can occur in the carb itself where the butterfly shaft works in the carb body (loose fit from wear).
You pretty much need a vacuum gauge to diagnose the problem. For $30 it's not a bad deal. You'll need it over and over again anyway as the years go by.
Rev
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'66 Coupe, 306, 350-375 HP, C-4, 13.07 e.t., 104.8 mph, 1/4 mi.
O.B.C. #2
'66 coupe
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