Personally, I think the mechanical fix is the easiest option. The basic gist of it is, you pick up two spark plug anti-foulers from you local auto parts store. Then, drill out the hole in one of them with a 1/2" drill bit. This will widen the hole enough for the tip of the o2 sensor to pass through. You screw the o2 sensor into one of the foulers and then screw both foulers into the cat/test pipe/etc. Since the o2 sensor is moved back from the direct exhaust flow, it "thinks" the readings it's getting are correct and the CEL is eliminated.

Since you're using a gutted stock cat, you may only need one fouler for this to work. There's a lot of information available online on building this system. Search around. Good luck.