It is still clear cut. You need the three steam holes on the bottom of the head to allow steam pockets which could form to escape. They do not always form, especially above 2,800 RPM, but when they do they do form they do not generally go away. Once formed the steam pocket doesn't conduct heat so the area of the block heats up and you can get into detonation.
The production heads had six holes (three 0.250" and three 0.125" holes) in the head, you only need the bottom three 0.250" diameter holes (exhaust side). They are not bored straight down, but at a 40° angle sloped towards the exhaust ports using the head gasket (Fel-Pro 1014) as a template.
Big Dave