It pretty much hollow around the cylinder walls. The rear drain port is pretty important to get free of crud. Once you have water coming nicely out of that port, with a good thermostat in, and I differ from others in that I always put in a 7 pound cap on the radiator. 7 pounds is almost nothing in the scheme of things but puts enough back pressure on the system to give it positive cooling flow. Its really a matter of your experiences one way or the other. If things are really bad, you can always remove the freeze plugs making sure you have a source for new ones, but at that point I would think its time for an engine rebuild.
