I believe the bigger bore engines and 4bbls(not sure about the a/c cars) got a fuel pump with more volumne. Since they delivered more fuel to the carb inlet than was needed at times the excess fuel was returned to the tank via an internal bypass in the pump and the return line. I don't know anybody personally that it's happened to so maybe it's just an urban legend but...I've heard it said that if too much back pressure is created on the fuel pump diaphram(ie., capped of return line) and it ruptures, your crankcase can fill with gas which can create several other issues. None of them good.
Under normal conditions one of the smaller pumps should deliver more than enough gas. I'm running an 840CFM qjet on a 400 and it would run out of gas at about the 1000ft mark. Just wasn't strong enough to pull it all the way from the tank and keep the carb full. Changed over to an electric and problem solved.