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I killed a WB sensor by placing to too close to the manifold on my N/A. A lot of people have no problem placing their WB in the elbow after the turbo though. You definitely want it before the cat, as close to the header as possible without it getting too hot. From asking around it's very common to place it in the downpipe right after it bends under the firewall (that's where mine was on my N/A setup and where it is on my turbo setup).

As for replacing the stock sensor or adding another... depends entirely on how you're setting up and tuning the car. Some setups require both, some require only the wideband, and others can use the wideband to supply a stock NB signal to the ECU. For your purposes you may replace the stock NB sensor with the WB and program one of the LC-1's (or whatever WB you go with, as long as it has programmable outputs or a NB output) to supply a NB signal to one of the two analog outputs and the other will supply a WB signal to the gauge.