The pressure limit also refers to optional heater. Perhaps even to seals in the water pump? Maybe others will say "do not " worry. I worry about anything outside of specs.
A 140 degree thermostat was standard - it was fully open at 170 degrees (alcohol coolant).
The highest temperature optional thermostat was 156 degree - it was fully open at 194 degrees (permanent anti-freeze).
Most likely a 0 pressure radiator cap?
http://chevy.oldcarmanualproject.com/chevyresto/4684.htm
