The old DC Performance Book recommended sleeving both lifter galleys with aluminum or copper tubes. You then "dented" each lifter bore so the lifter would slide in and drilled out all the intersecting oil passages. This was done to allow roller lifters as the roller lifters of the day uncovered the oil passage at max lift and you lost oil pressure. The procedure and reasoning for it is in the book. This is what is sounds like someone started to do on yours.