I don't believe it's possible for the gas to drain back to the tank. First off, there should be a black spring loaded anti-siphon valve on the end of the fuel filter. It takes fuel pump pressure to get gas past it. That is for in case the car rolls over and so draining fuel won't feed a fire. Second, the fuel pump won't allow fuel to flow backwards. If it did, you would be starting up dry every time the car is shut off. Thirdly, when you disconnect the fuel line, does the bowl empty? No, of course not, the float valve keeps it in. If it was stuck open, you would be flooding like crazy when the motor was running. More likely it is evaporating or leaking. Alcohol in the gas makes it evaporate fast but overnight is a bit much. Leaking is the bugaboo of Rochester's, but was supposedly solved around 1971 when a better way of plugging the main wells cured that condition. That does not mean yours can't leak, as it probably is. Another possibility is that the exhaust crossover that passes under the carb is getting hot exhaust all the time due to a stuck EFE valve and is boiling the fuel. In that situation, the crossover gets red hot from being on all the time and when the car is shut off, the heat evaporates all the gas out of the carb.
