Earl,
Somewhere back a long time ago I read an article where a Cobra owner described a Ford tractor in hose thermostat in his car’s upper radiator hose in the late 1960s or maybe early 1970s. Sounded interesting. When we installed the play engine in our red car I bought a new old stock Ford tractor thermostat. It was indicated that it started opening at 160°F, which was the stock Cobra unit rating. I tested it in slowly heating stirred hot water and it started opening right at 160°F. I made two modifications. The outer diameter has a narrow flange formed in for the tractor application that I removed. With the flange gone the unit fit inside the hose very well. The second was add a ring of tiny drilled holes around the valve seat near the tube wall to let air or coolant through. As made the valve was a pretty good seal. My ring of holes helped get the air out during first fill and lets a little water circulate all the time to bring warming water to the thermostat.
I think it works pretty well I think, even when we were caught in an unpredicted snow storm during a tour in the Rocky Mountains. The only external evidence something extra is in the circuit is a hose clamp where the thermostat is. I will add, that our heater is connected and working so some coolant is always passing through that heat exchanger also. I would think that engine warm up would be a little quicker without a heater side circuit.