you are confusing a vapor line with a fuel return line the gas goes through the pump and the excess goes back through the smaller line if the line is not there, then the pump will put pressure on the float. if the float is weak, the gas will continue to fill the bowl till it over flows. the reason it fills up slowly is the 67 and 68s had no vapor release so the gas will back up through the tank and out on to the body or (burping).
either you have a bad float or the system is incorrectly set up. give h-ll over there!! stay safe.