The correct 261 bypass thermostat has been out of production for many years. There is an extra port on the 261 housing to route coolant back to the water pump during warmup that the original stat blocked off with a flap valve once it opened up fully. Once the engine reached full operating temperature, that setup became a moot point. The only way to determine if the bypass holes in the head have been drilled is to pull the head. They're there to improve coolant flow beside the three pairs of siamese-cast cylinders, so if they haven't been drilled, the block, not the head would be too hot. Unless the coolant is boiling out of the radiator, the engine IS NOT overheating. Your temp difference between block and head is completely normal. Top to bottom temp difference at the radiator should be 20-30 degrees F. at least. It looks like you need to install a bigger radiator, or do a good clean and flush on the one you have.
Jerry

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