I don't bud. If I understand correctly, you installed just the upstream sensor, the CEL went away, but then it came back, it is the same CEL code as before, and the one thing that started all if it was installing a header. Is that accurate? I would pull the sensor and see if it is covered in sooty crap first, if it is I'd clean it and clear the code. I would think that either the sensor is getting fouled with crud by an engine that's not running properly, or there is an electrical issue with something related. I would think that timing or fuel management could be at the root, but you aren't getting any codes for that and it is running smooth right? Wish I could be of more help.