You can't drill the body of a true high flow thermostat, it would ruin it. The oem 351C thermostat is a true high flow thermostat, manufactured by Robertshaw. In other words the 351C came with a high flow thermostat from the factory, no other American V8 did in that era. The opening of the oem thermostat is much larger than the opening of a conventional thermostat, like the Windsor thermostat for instance.
Unless the proper thermostat is installed in a 351C, a tremendous amount of hot coolant bypasses the radiator and recirculates through the motor uncooled. This will lead to overheating problems every time.
-G
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