Sounds like a good idea Cooperman! Someone recommended caustic soda years ago but I've never tried it although I've found that reverse flushing help.
Never, ever, use caustic soda if there's any aluminium anywhere in the cooling circuit. You might have a cast iron head & block but don't forget thermostat & water-pump housings and the radiator.
It used to be used to decoke engines but problems became clear when aluminium became common in air-cooled engines (most motorcycles) in the 1950s. Stay safe and used a solvent designed for the job
