I have noticed on SBC engines similar to mine the exact modification you are refering to. I have a set of AFR227's that I added additional coolant fittings to in the area between the center two exhaust ports just above the head bolt flange with -8 supplied directly from the electric water pump body proir to it's engine bolt flange. I personally would not attempt this mod on the head without being able to inspect the water passage from the deck mating surface to determine the least intrusive spot to drill without comprimising it's integrity.
Cause: provide coolant directly to this HOT spot that has not been preheated from passing through the cylinder wall jackets.
Effect: Lower temperature coolant is able to absorb additional heat taking it away from the HOT spot.
This was applied to a 13.75 to 1 compression motor that see's ALOT of street driving with NO overheating problems and seems to stabilize coolant temp's between cyl's. This was evident by the absence of the dark spot usually found on the head gasket between the center cyl's. Hope this helps.