I always put the water temp sending unit into the heater hose as it comes out of the head. That way its measuring the heat of the water as it comes out of the engine and not picking up the heat of the manifold itself.
This is assuming you don't have a dead head heater hose setup. Older vehicles just block the heater hose when your not using the heater. Newer ones bypass the heater core but allow the water to continue to flow back into the motor which is IIRC how it is on your X.
~Mark