You probaley know this, but I don't know if you know. The location of the PCV on the 304? And it does go to one of the larger ports on the carb. There should be enough vaccume, that when unplugged, the engine wants to stop running. You are correct about the oil fill cap vent, this were the fresh air is drawn into the engine.
The only reason for oil to come out the tube, is internal pressure in the crankcase,
1957 WILLYS pickup,
1973 J 4000,
1978 CJ7 DD.
1979 CJ7 360, TH400/Quadratrac.
1980 CJ5 trail Jeep.
1989 YJ the CJ to YJ.