360 Tune Up

With the 5 speed gearing, the engine is running relatively slow, but not sure that's of any significance since there were overdrive 4 speeds and highway geared cars.

Accurate air temperature measurements are difficult.
Conductive temperature measurements aren't so difficult. If you have a multimeter with thermocouple or some thing like that you could clamp it to the radiator outlet.
Do a feezing water, and more important a boiling water check so you know roughly the accuracy. most termperature measuring devices are repeatable but not too accurate unless calibrated.
Anyway the point here is to get a sense of how much radiator is dropping the coolant temperature.