You can use the rear o2 bung, but the values will be slightly different from doing it more upstream. I ended up having another bung welded into the downpipe portion for $10 since I am catted and using both narrowbands
And yes, you can use the same gauge/sensor as of their 4.2 sensor. check out the site if you want, under key features
http://www.aemelectronics.com/wideba...fuel-gauge-25/