I would not suggest replacing any of your factory O2's. The front 2 are needed for tuning, and the rear 2, if messed with, will throw a check engine light (unless disabled in the tune). Even if they were disabled, I wouldn't put my wideband O2 that far back in the exhaust. Best practice is to weld in a new threaded bung for the new O2. Any exhaust shop should be able to do this if you can't do it yourself.
Your intake manifold should have a place to run a boost gauge off of. The wiring for the wideband should be pretty well described in the instructions for the gauge you purchased. For +12v, I tapped into the cigarette lighter on my car I believe. The one above the radio.
