Another clean-as-you-drive product is the Schaeffers #258 Clean & Cool. Albeit harder to get, it has worked great for me. At least visually, the tops of the rad tubes, manifolds where the hoses connect (insides) and the thermostat/housing are free of any deposits or slime. It also stabilizes the coolant over time to prevent it from acidifying. Might work well in your case if you have the time to drive it around awhile.
I too subscribe to the several fill/drains with just distilled water. I make sure to disconnect the lower hose, open the petcock on the rad and open the drain bolt on the engine block to get out as much as possible. Parking the car nose down the driveway adds alittle gravity assist with the drain too. I do have to turn it around on the fill though to bleed out the air. I always measure how much comes out on the final drain and subtract that from the total capacity. This gives me a general idea how much DH2O is still in there and add/mix the coolant accordingly.