I've got a header pipe for a car with the O2 in the CAT. I've been looking around for a header that will work with a manifold with the O2 in it but haven't found any. I was thinking just now about sending the header back and finding the right one for my manifold. But my O2 sensor will not come out of the manifold due to the manifold blocking any access with multiple wrenches and sockets. (can't figure that one)

Anyways if I switch out the manifold which I assume should work just fine and replaced the O2 sensor in the Cat will my ECU have any difficulty reading the O2? Would it be better to put it in the header near the Cat so it reads more like if it was in the manifold.

Any thoughts are welcomed, thanks.