As Dave suggest. Install a water temp adapter on the radiator hose..( good idea to know the internal size of the radiator hose, so you get the appropriate water temp adapter..34mm..36mm..)
But my view in the placement ..
TOP HOSE. will tell you the 'already cooled' water leaving the radiator back to the engine head.
However..the BOTTOM HOSE..which is after the thermostat..will show you closest too actual engine temp..and if your gauge has a controller. You can set the fan to activate from your setpoint..and most cars have a temp unit in the bottom hose circuit..not the top..
Unless you get a smaller adapter and put the temp unit on the bypass hose from the head to the firewall AC heater..that's actual engine head temp..
Hope this makes sense and helps..