Tuning after headers??

A double pumper will be too much and harder to tune, especially on a mild 318.
Find out what the carb should have from the factory and what it has now. You can look it up by the carb # on the choke. Then go bigger on the squirters (for "immediate" throttle response) and might have to go bigger on the pump cams if there is a flat spot after roughly the first couple seconds. Gotta check the pump arm adjustments with any pump cam changes. You may have to buy some squirters, pump cams and gasket kits.

Headers may have leaned it out, on the pump circuit as well as the jets.

Hopefully the secondaries aren't opening too quick.

It would help to find a book on Holleys to help you understand and tune them. The books published by HP Books has helped me a lot. They have pump cam charts also, but the carb numbers,carb specs and pump cam specs can be found online.

Gotta go - work starts early.