Most EGT probes are 1/8" NPT.
If your careful, and use some grease, it CAN be done without taking your manifold off. but if you like the hotside of your turbo, i would take the manifold off.
An EGT gauge should go on your HOTTEST runner. Likely #3 in our case, since the flow is greatest on that runner of the Intake manifold. Thats just a guess on my part based on MAPs flow research.
-Sean