68 383 with 750 carb. Runs rich

YR's assessment sounds pretty good.
My engine cruised at 15. And with the PV plugged, it would go flat lean at 10.5 on the gauge. The closest PV I had to that was a 12.5, and it works pretty good. Been in there since year 2000.
Using Holleys method, I wouldda had a 5.0 in there. Or a 4.5