If you don't want CEL use one of these or equlivant, you might have to try the different orifaces to find which stops the CEL best
http://www.phasteksport.com/2013-Sci...-o2-020001.htm
Moving the position of the O2 sensors is likely to cause additional problems especially if you move the front o2 sensor
You can disable the CEL code on the stock tune just open the stock tune in RomRaider set the P0420 CEL to disable save reflash stock tune.
It kind of pointless though as
1. Its no longer byte for byte stock
2. The Dealer will notice your aftermarket headers WAY before they will notice the Hex code changes in a tune
If you have aftermarket headers fitted your chances of engine warantee claims are reduced, same as if you have messed with position of O2 sensors.
Headers change gass flows
Re-position O2 sensor -reads mixtures differently.
They can argue you damaged O2 sensors causing problems
They can argue you introduced exhaust leaks or leaks at o2 sensors that caused problems
Any melting or heat deterioration of plastic parts belts ect can be blamed on lack of heat shielding of aftermarket headers.
your aftermarket headers did not have expansion joints causing stress on block ect list goes on.
Aftermarket headers stand out like dogs balls
Most dealers will never find a tune unless they have other evidence or reason to look. The ROM will show the same CALID or ROM ID for tuned or stock ROM in Techstream Dealer software. They would have to pull your ROM and compare it with HEX comparator or other software to be sure and that ain't going to happen unless they are after you for some reason