1979 ford f150 sludge/sediment in cooling system?

my truck runs hot at speeds above 60. Cooling system has brown/red sludge all in it. I have flushed radiator 3-4 times. I have ran radiator cleaner through it. I hve taken the radiator out and flushed it from every opening. I have replaced thermostat. Truck still overheats and has sludge in system. Help

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