your taking about installing it into the water jacket right? You can do it while its still on the car. you got to drill all the way through and tap it. Remove the thermostat neck and enough coolant, than stuff a clean rag or something down there so when you drill through, metal shavings dont fall in. it has to be in direct contact with coolant or else it wont work.

http://i36.tinypic.com/2yungk0.jpg

Hypertek my friend, u have just solved my problem of where to put the sensor. I was looking at that earlier but i wasnt sure if i had clearance and didnt want to screw anything up.