I know you probably won't like this recommendation, but here it is anyway. lol You might kill two birds with one stone. I would try a 2 into 1 pipe at the ends of each 3 into 1 maybe back like 36-40". That picks up torque like a big dog, plus you would have a more accurate place for the O2 sensor. I can tell you from first hand experience very recently on Vixen. When I put the wye pipe on, it was a total night and day difference from open headers.