I use a bottom spacer, then the heat shield, then another spacer on top. I have always had them on in this order. So far this Spring, the fuel in the bowls only rises about 1/16" during the heat soak process when the engine is shut off with the hood closed. When I first installed them, I would check the fuel level every five minutes to see how high the fuel would rise in the bowls. Since running this setup, the fuel only rises about 1/16".
When the engine is running or after a drive when the engine is idling, I can feel the carb and it is lightly warm at worst case. These Cool Carb spacers and heat shields are good to 300 degrees. I run a 195 degree thermostat and I have not had any issues with these items this Spring. The summer will be a good test though.