Cracked block, huh... what else is the engine doing....like loss of power? weird noises? etc. Run a compression check across all 6 and check the carb. Too lean of a mix can make an engine run hotter...so I have been told by several mechanics. You can also remove the radiator cap when the engine is cold, start/run it and as the thermostate opens, observe the water flow rate. Also check the timming. and advance.
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