@dgibby,
I'm trying to resolve a cooling problem on my Dart and was closely reviewing your pictures and I think you might have the thermostats identified backwards.
I have both thermostats and my Milodon 180 degree is silver and my Stewart Components is brass and has the telltale Stewart modified 3/16 holes drilled in the poppet valve. You can just barely see the drilled hole in the poppet in your second picture on the brass thermostat you've identified as the Milodon.
Would you please recheck the thermostats?
