IMO if drilling straight in would reliably intersect the water jacket GM would have done it that way. Take a look at why even a slight angle towards the manifold side will improve chances of flow. Note that 3 of the holes are pretty much can’t miss and the other 3 may not exit the head bolt boss if drilled straight. I’m sure there are folks out there that only have flow in 3 of 6 places but possibly that is just enough to get by. GM locations are in green and red shows straight in.
