I have used automatic dishwasher powder with some success. Just put a couple of cup fulls in the radiator and fill it with hot water and leave for a day or so then drain and flush. I have heard one report of it wearing out the the water pump thrust washer when the engine was run for too long as this stuff is abrasive but it sure works. With the early chevs (pre 37) it is important to put soluble oil in the radiator to lubricate the water pump - you should also use a special soluble water pump grease in the water pump as normal chassis grease will block your radiator.

Ken

:p The radiators back in and she's running