You need to go to an exhaust place, and have them weld in a bung above your cat. once the bung is welded in, you can screw the sensor into that, and run the line up thru the firewall and into the cabin, to the gauge.
stage 2 does not use 2nd o2 sensor. You can take it out the hole (but it must remain plugged into the connector) and just ziptie it to the side. Then screw in your wideband o2 sensor in there. You can also get a MAxxfab down pipe & borla catback for very cheap - or you can even connect it directly to your stocker -- just make sure you get rid of the cat.
The problem being the downstream O2 sensor is in the cat. The A/F ratio will have been modified some by passing through the front half of the cat when it gets to your wideband sensor, thus giving an erroneous reading.
I would follow the excellent advice from Kustomatix! :thumbsup: