The holes in the head gasket are the size they should be. If they are opened up water wouldn't flow to the back of the engine. It would come out of the pump and go right up the first set of holes in the head on both sides and through the thermostat and back to the radiator. The back of the block and heads would get no cooling.
The holes in the block and heads are huge so they could get the casting sand out when they poured the block. Yep, it's that simple...
As far as the slow water cools better I disagree.
This is from folks who do nothing but make water pumps for engines that cost more than most people make in a year..Stewart components
"A common misconception is that if coolant flows too quickly through the system, that it will not have time to cool properly. However the cooling system is a closed loop, so if you are keeping the coolant in the radiator longer to allow it to cool, you are also allowing it to stay in the engine longer, which increases coolant temperatures. Coolant in the engine will actually boil away from critical heat areas within the cooling system if not forced through the cooling system at a sufficiently high velocity. This situation is a common cause of so-called "hot spots", which can lead to failures."
You can read the whole thing here: http://www.stewartcomponents.com/tec...ech_Tips_3.htm
Engines don't overheat everywhere at once. they start to boil away coolant in hot areas first. On a SBC between the center two cylinders where the exhaust ports are next to each other is usually the first place. Once the coolant boils away in a small area the gases form a pocket that keep coolant from contact with the metal making it hotter, boiling away more coolant making a bigger gas pocket and on and on.
Thats why on engines that run hard for long periods it's common to route cooling directly to the head at this point and sometimes straight to the back of the block.