Hi, Guest
I did some WOT logs of sensor before and after CAT and it was about 2/10ths off the A/F. Wasn't really that bad, this is once the engine was warm. Before it was warmed up the wideband readouts looked way off.
What I would do is use a blockoffplate for the EGR, grind off the EGR port on the CAT and weld in a bung there for the wideband. I don't have a pic of the stock cat here but that to me is the easiest. I think in the header is real tight to put a bung in to get accurate readings. you want it slightly down from the connector to ensure your reading the a/f from all cylinders.
Also FYI: SVT header will not work well at the bottom flange, due to the collector design down there.
What I would do is use a blockoffplate for the EGR, grind off the EGR port on the CAT and weld in a bung there for the wideband. I don't have a pic of the stock cat here but that to me is the easiest. I think in the header is real tight to put a bung in to get accurate readings. you want it slightly down from the connector to ensure your reading the a/f from all cylinders.
Also FYI: SVT header will not work well at the bottom flange, due to the collector design down there.