It sounds like the gas is draining out of the carburetor due to a leaking or defective power valve. Take the carb off, remove the air horn, sit it in a pan, and fill the bowl with gasoline. Put a piece of paper or aluminum foil over it to minimize evaporation. Check the fuel level the next morning. If it's down significantly, you've found at least part of your problem.
Even if the carburetor is new or rebuilt, one backfire can damage the power valve. Also, there is a chance that the power valve may not be correct for your carburetor. Later model Holley's used a power valve the looks almost exactly like the proper one for an early carb, but the transition between the threaded part and the body is different, which can also lead to leaks.