Ok, I just replaced everything forward of the factory muffler on my stock svx and can recomend the set up I went with.
- 2.25" tubing all the way from the flanges that bolt to exhaust manifolds. Cut the factory tubing off at the flanges and reuse the flanges. The tubing in front of the flange that slides into the exhaust manifolds and is surrounded by a copper o-ring is much larger than the tubing after the flanges--it's something around 2.25".
- oxygen sensor bungs in the factory locations
- magnaflow/carsound 940xx cats in the factory primary locations
- an 18" long dual in single out magnaflow muffler in the location where the factory dual inlet cat is
- 2.25" tubing back to the factory muffler
That set up has absolutely zero resonance or under car noise and is quieter than the factory set up.