340 engine / oil question

Anyone? I went to change oil in my 340, pulled oil filter off and no oil ran out all over the headers like it allways did, checked the oil filter and it only had about 1/3 of a cup oil in it. Put on new filter, 4qts oil, ran engine,oil pressure was 55psi, after it warmed up it droped back to 22psi at a stop light and 56psi running 45mph. Then I checked oil, it showed that it was 1/2 qt low. That's not right , should be 1 qt low. After the engine cooled I removed the oil filter and , it only had about 1/3 cup oil in it. I also hear a little valve rattle running about 35-45 mph. Any help? :cussing: Oh, The oil pan in a after market pan, could that be to short and blocking the oil pickup on the oilpump?