I had a thermo switch to turn on my fan in my El Camino. 327 motor. I used the type of switch that is supposed to be installed between the radiator fins. I cut off the probe and sanded it flat. I then JB welded the switch to the top tank. It worked great there. Fan would turn on and off when it was supposed to. I think it was on at 200 and off at 190. Its been a while. 180 tstat. Good luck