The thermostat manufacturer controls/guarantees at what temperature the thermostat will start to open.
The 1954 shop manual says a 180 thermostat starts to open at 174-184. A 160 at 157-65.
The fully open temperature can vary a lot and can typically be 10 to 30 degrees above start temperature.
GM typically installed 160 at the factory (covers a larger range from the hot desert to cold mountains).
180 is also fine on these old engines.
A 195 thermostat is only for today's fuel efficient, low emissions and high tech automobiles.
