416 stroker temp?
That is dead rich and who can tell if the plug is too hot with the black shell.
As for your heating issue...there is no way if the system is capable of cooling the engine that you should be anywhere near 200 ever if you are using a 160 thermostat. You have issues. Why those issues are cropping up now I can't say.
My bet is you are turning the water pump way too slow. It should be overdriven by ~10% at least. There is no reason to have that much differential between thermostat opening and operating temperature.
