best coolant flush?

I'm just looking for a way to get my cooling system squeaky clean as it just turns coolant brow. yes I run distilled water antifreeze 50/50 mix. This is on my '74 D100 with the stock radiator. Shoulda done this before summer, but stupid is as stupid does. This is my daily driver so I'd like something that isn't gonna wreak havoc or cost mucho moneys.