Here's another way of tapping into your coolant to get the temp for a temp gauge:
Pick one of the radiator hoses (upper preferred), splice it, then insert a section metal pipe that is the same outer diameter as the inner diameter of the radiator hose. The pipe would be drilled & tapped already so that a temp sensor can be screwed into it. I have also seen some weld a "boss" to the pipe so that the temp sensor can be screwed into the boss on the pipe. Before inserting the temp sensor into the pipe, apply silicone or liquid teflon to the threads on the pipe (or temp sensor). Screw the sensor into the section of metal pipe. Reinsert the small section of metal pipe between the splice on the chosen radiator hose and use (2) clamps to secure the hose & pipe.
Sometimes doing the above is easier than tapping a thermostat housing, especially if it's quite a bit of work just to remove the thermostat housing (or trying to access some other area to tap into the cooling system).
The most accurate reading will be that of a temp sensor that makes contact with the coolant fluid, rather than one that is attached to exterior metal surfaces (or sandwhiched into a radiator's fins).
Maybe the above is helpful to some, but that method is used by many folks in racing (drag, auto crossing, road racing, etc) and does not cause any detrimental affects at all.