I've actually had head bolts blocking the oil feed holes down by the transfer slot in the head, so that oil won't get to the rockershafts, but have never seen a head bolt completely blocking a drain hole. However, usually I will open up those drain holes a little where the head bolt goes by, to let the oil drain back a little easier. There isn't a lot of room there in some cases. If I recall correctly the early Edelbrock heads had some issues with this, and later heads opened up the size of the drain hole somewhat. Can't remember for sure. In any case I have gone in there with a die grinder and a burr and just made the passage around the head bolt a little bigger.