I used a 2.5 inch flange for my o2 housing, shrank the top of the downpipe to 2.5'' and then went from there, it was all one piece. I had him order all my mandrel bends since he couldnt do it without it being crushbent. My muffler is what cost the most for me. With my new exhaust, it will be all 4 inch with a dynomax bullet muffler in the of the car. Ill be doing it my self though.