I've used a piece of steel cable in a drill motor to clear the crud out of a petcock drain port, but that's only the beginning. Once the coolant will drain, you'll need to circulate a very aggressive cleaner like phosphoric acid or Trisodium Phosphate (one at a time, not together) to clean the crud out of the bottom of the block. The acid dissolves rust and other metallic contaminants. The TSP cuts greasy deposits. Both chemicals are available on Ebay and they can be mixed in much stronger concentrations than are available in commercial flush products. It will be a long, tedious process to get the block really clean.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!