Here is show a engine block cut in half and you are looking at the oiling to the mains from the large oil galley that runs down either side of the lifter bore BUT ONLY oils the mains on the passenger side. When i drill mains out i use a .337 drill this makes the the main oiling hole larger by.018 it may not seem like much but it REALLY makes a difference. Not all blocks are born equal! ;) This is a pre 78 360 block (The thicker version)

here are the pics