Large rpm drop

Charlesvolare said
idle mixture screws 1 turn out from seated, fuel pressure at 6psi, float level checked,...

Mattax said
Pressure and float level are good to know.
These should result in the proper fuel level.
The way to check that is at the site plugs. Depending on the carb, fuel should either be just below the plug, or in the mid area of the glass.

Are they set it as shown on Page 11 or 12?
https://documents.holley.com/199r9934-3rev4.pdf
or as much as 3/32" lower than Holley showns on those pages is OK- esp secondaries.

If the fuel is too high in the bowls, then the mains can easily be dribbling. And if they are much too high, fuel will come out of the boosters until the fuel pump shuts off and the bowls empty to a point lower than the top of booster passage.