Is the thermostat the type with the standard opening or an oversized one?
The version with the larger opening may allow better flow out of the engine into the top of the radiator. The side of the thermostat with the coiled spring faces into the intake manifold.
https://www.ctci.org/1957-thermostat/
https://www.classictbird.com/Thermos...uctinfo/8575C/