Is the cup plug missing above the rear main cap, in the galley to make the oil go through the filter? You can check to see if it's in there by pulling the oil sender in the rear of the block and inserting a piece of welding rod down the hole. I checked 2 blocks in the garage and the rod went in 7.5" from the top. If it's not in there, you'll have low oil pressure, and it needs to go in from the bottom.