How close to a H-Pipe can I be with my oxygen sensors? Currently I dont have any cats and my O2 sensors are mounted 6" past the 4 pipe header collector.

I believe H-Pipes need to be fairly close to the collectors to work properly, but I dont want to mess with my tuning. I would think having it too close would skew the readings somehow. Anyone with first hand experience?

Also, I have true dual 2.5" exhaust right now. How much smaller does one make the h-pipe?