Main issue was that both radiator hoses connect to the top of the engine, so draining the radiator will get nearly nothing out of the block not to mention the heater cores. Yes it had drain valves on the block, but I wasn't about to risk snapping them off on a 1/2 million mile engine and creating a much worse problem. So for a few cycles of drain, fill, drive drain, I was only able to add about one gallon of water at time and the stuff draining out was not getting visibly cleaner.
Checked on the Toyota forums and found that you could disconnect the oil cooler water hose at the filter, and that would drain the block better then the drain valve. So I did that and was able to add two gallons of water at a time. Did that half a dozen times until finally the water draining out was clean. Refilled with 2 full gallons of extended life Peak Asian red/pink concentrate. This is the only full strength extended life coolant made that was available at a reasonable cost. Will check freeze protection after a few drive cycles to get it fully circulated, but with a total capacity of 14 quarts, it should be better than 50% concentration.