Use this thermostat spacer (or similar):
Thermostat spacer with EFI sensor port | AR Engineering
Putting it in the lower radiator hose gives you the temperature of cool water exiting the radiator, not engine temp.
Putting it in the upper radiator hose puts your sensor on the wrong side of the thermostat, if your t-stat ever fails (closed), you'll never know it until after you cook your engine.
You want to keep it in the engine side of the thermostat, similar to where the factory mounted it, so you get a true max "engine temp" of the coolant as it's about to exit the engine.
