Quoting galant1517:
The first question would be, do you still have your emissions stuff intact? The reason being is that the easiest place to install the probe is where the thermo-vacuum switch is on the t-stat housing. Autometer makes metric adapters for those probes, or you can tap that hole to a 3/8" npt if that adapter is included. Otherwise I'd say your choices are somewhat limited. The t-stat housing is fairly cramped up in all stock form, not a lot of extra room if you are still feeding coolant to the turbo and the sandwich oil cooler and the throttle body. I'd say the margin of error is too great to try and tap the head directly, same with the block. Which leaves us with the water pump or the water pipe, neither of which are very accessible. You could cobble up a brass tee with hose fittings on opposite sides and the probe running perpendicular to the hose and put it in line with the heater hose coming from the t-stat housing I guess.
This hose constantly receives engine temperature coolant at all times. The upper radiator hose tee would read low until the t-stat opened, and probably would fluctuate 10-20 degrees while warm due to normal t-stat operation.