If you take the compensating board out, you already have a coolant temp gauge.

When you get your turbo cluster you might wonder what to do with the turbo gauge. I decided to use it to monitor the coolant pressure in my 84 normally aspirated 240. It turned out to be a very fortunate decision for me, as shortly afterward I lost the water pump gasket near the end of an 800 mile trip in the fast lane of an 8 or 10 lane expressway. It was another stroke of luck I was watching the pressure gauge, because it went to zero giving me just enough time to find the next exit before losing enough coolant to make the temp gauge rise while it peed coolant onto a parking lot. A third stroke of luck (I'm very lucky) occurred when I found a still open foreign car parts place nearby willing to cut loose a gasket kit from a new water pump. Anyway, I credit that turbo gauge with saving the old head with 300K on it at the time.
Tapped into the highest part of the cooling system.