You'll need to consider fine tuning your jetting once you determine whether you want to optimize open headers or full exhaust. There will likely be a difference. In my particular 360 application, going from 3" open headers & collectors to 3" X-pipe & straight thru bullet mufflers, it ran even better when jetted down 2 #'s all around, I'm assuming that the X-pipe helped efficiency. Your results may vary.