My idea is to take the stock flange on the manifold open it up to a little past 2.5 weld in a 2.5 flex to that and build a straight as i can make it 2.5. I can get it real close too the floor i am deleting the 02 so wont have to deal with that. I had another regal gs and i did something similair it was all stock except exhaust and CAI.