Compression check. Cyl 1-130, 3-145, 5-148, cyls 2,4,&6 all 150. Engine runs great to listen to it, 230,000 mi., no water in oil. The problem is, when thermostat opens coolant boils in tank and pushes out overflow. Engine temp gauge reads normal or below. If car is driven, coolant reaches the level to cause overheating. Cooling fans do not even come on as coolant begins to boil over. It seems to me that block sealer would not be able to bridge this gap. Greatly appreciate your feedback.

Either you are losing coolant from an external leak, or from an internal leak(intake gasket, head gasket, etc,) which causes an air pocket and an overheating problem....one other thing to check is the heater core hoses, are both hot? If the output hose is much cooler, try back flushing the heater core...

Rule of thumb on compression, if lowest cylinder is is around 10% of the highest cylinder, then compression is usually ok....

As far as the head gasket goes, get the engine warm, just before it overheats....safely remove the radiator cap(if none, then the pressurized reservoir cap), and attach a coolant pressure tester....pressurize and leave overnight....next day disable fuel and spark, and remove plugs....crank over vehicle and if coolant come out of the plug holes, you have a head gasket problem...

Another good question to consider is whether or not the cooling system is holding proper pressure under normal operating conditions to begin with. When you figure that normal operating temperature could be in the same range as the typical water only boiling point, it'* very easy to run in to a recurring issue, especially if there is air trapped behind the thermostat, delaying its opening.

Coolant and pressure raise this boiling point, thus preventing any issues under normal circumstances.