before cats if you want the most accurate reading. I like Ihiryu's y pipe plan, as it makes better use of the 1 sensor. You could relocate the cats a few inches down stream to allow a piece of straight pipe between the stock cats and manifolds and then place it there. If you can deal with spending about 400 on 2 sensors, this would give you the most accurate setup. 1 day bought my first sensor and installed 1 side, the next month I bought the other side due to the double price. A single sensor is ok, but remember that your blending the afr of both banks together so if an injector one 1 side sticks open, it wont be as obvious on the display. Having the #2 sensor/display wil be almost like a flashing light when you know both screens should be identical when the engine is running properly.