Running without a thermostat ok?

If it overheats with a properly functioning thermostat, it will overheat without one. If the thermostat does not open, replace it. Changing back to a 160 shouldn't make any difference. Since the engine cools with the car moving, it would seem that the radiator and thermostat are functioning properly. Heating up in traffic on a hot day isn't abnormal, and 200 is just fine. Take a good look at the fan and fan clutch, and fit a shroud if not equipped.