Yes, you need a thermostat. The reason for the 160 degree stat years ago was for engines using alcohol-based antifreeze. Any hotter minimum temperature would evaporate the alcohol. When ethylene glycol-based antifreeze got popular, the standard stat became the 180-degree type, since ethylene glycol has a higher boiling point, similar to water.
Today's thermostats are usually in the 195 degree range, give or take a few degrees, because hotter-running engines tend to be more fuel-efficient when combined with high-pressure radiator cores and 16-21 lb. pressure caps. The proper thermostat/cap/radiator combination for mid-50's and newer stovebolts would be a 160 or 180 stat, and a 7-lb. radiator cap. Be sure the radiator is clean and free-flowing.
It's possible to flow coolant too fast through a clean radiator, resulting in not enough time for heat transfer from the coolant to the air. I used to install a flat washer with a hole about 3/4" in diameter on my race engines instead of a thermostat to control coolant flow.
Jerry
