M20 thermostat housing is notorious for being the highest point- and any leaks causes air to get onto the sensor giving you a false LOW reading.
You can put a gauge in the back of the block at the outlet where coolant used to go to the throttle body. There is also the return for that coolant into the bottom of the throttle body, which can be tapped. This will give a reading around 160f when the engine is operating normally (80c = 176f)
The best place to put it is honestly the lower radiator hose, because that will give you the highest temp. Not really long enough to put a temp sensor insert though.
My temp sensor is still in the thermostat, with an adapter. it works fine.
A pressure sensor is right next to it, to alert if there is any potential air trapped up there.