What you might consider, rather than just the block drain plug, is to take the lower radiator hose off at the radiator, remove the upper hose and thermostat w/housing, let things drain out, then flush through the water pump, into the block/heads, and out the lower radiator hose. IF you might use any cooling system chemical flush agent, use it weakly as the stuff it might remove could be keeping a hole in a freeze plug covered.
One other thing to consider is that the engine sits in the car with the rear cylinders a little bit lower than the front ones. The rear is where the sediment will collect first, not the front. Try to not use high pressure water as you're basically doing a reverse flush, so gentle water would be best, so it doesn't try to move something backward and create a blockage.
Might just put water in the system, run the engine until it's warm, then drain everything through the bottom radiator hose. Reconnect and re-fill with water. Cycle repeat until it runs pretty clear. Might even run it a few days in normal car use, rather than doing it all at once. Play it by ear and see how it goes.
When you're comfortable with the clarity of the water, drain the radiator and put 2+ gallons of quality coolant in the radiator. See how it goes . . . Also, be sure to dispose of the fluids in an environmentally legal/friendly manner.
CBODY67
