Your 600 CFM Holley likely has an electric choke. They work good, but, one drawback of electric chokes is they cool and reset faster than the engine cools. After sitting an hour or two the choke might be closed again, then plus some pumping of the throttle causes the engine to flood. One other possibility is when sitting for an hour or two with a hot engine, the fuel in the carb bowls heats up, percolates and drips into the motor through the circuits inside the carb, thus, flooding the motor. One of those thick heat insulating gaskets or a phenolic carb spacer will usually stop that from occurring. The latter situation is more common when aluminum intakes are installed and the heat crossover passage is not blocked. Another much less common cause of flooding with Holley carbs when the motor sits is leaking metering block gaskets in the carburetor.