I'm going to change the coolant on a '96 B-Body with an LT1 engine. It's got expired Dex-Cool in it (past the rated 5 year life). I want to get all the old coolant out in case it has turned to the dark side. Opening the drain on the radiator will leave some coolant in the block. I could pull the knock sensors to drain the block, but that will be a pain. The usual procedure seems to be to take off a hose and run the engine to pump the coolant out, replacing it with water through the overflow tank. Does I need to keep an eye on the temperature of the water going into the overflow tank? Is it possible to crack the block if the water is too cold?
Cold water will not crack the block. The hose procedure is common and you just have to be sure that you add the correct amount of antifreeze and water (preferably demineralized) when you are finished flushing.
Remember that automobiles in colder climates sometimes start when their coolant is below freezing. Granted both the engine and the coolant warm up together, but an iron or aluminum block would take a quite large, instantaneous temperature drop that cold water is not going to provide.