What would continue to cause overheating water in overflow tank boils after a five minute drive in an 82 Buick after replacing the thermostat and fan clutch?

If the engine has been overheating there is a real possibility that a head has warped, causing head gasket failure. If it's boiling over that soon after starting the engine, you should be supicious of a head gasket. You should be able to check the coolant for carbon monoxide as a test for head gasket failure. Test kits are generally available at auto parts retailers.