You may already know this, but a thermostat does not establish the top range of temperature your engine will run at...it establishes the lowest temp your engine will run at. I find that confuses some folks. I'd buy a 160 degree thermostat and go with it. If you are tempted to purchase one of the thermostats sold under the name "Fail Safe" please consider another brand. If your engine ever overheats---even once, that thermostat is forever disabled and left in the open position.

If you put a 180 degree thermostat in a pan of hot water with a candy thermometer, you'll see it starts to open at 180...just a crack and it may even be hard to see it is doing anything. It won't get fully open until around 220~230 and that's what yours is doing. I don't see any reason to run a 235 at 200 degrees or more and neither did the guys who designed it. Good luck!

Side note: a year ago I tested 4 different makes of thermostats (all 160 degrees) to see if they were all about the same. No dice. Two of them didn't begin opening until 180 so I assume they were either defective or marked incorrectly in the country of their manufacture (which was not the USA).

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end