you want hem to be before the mufflers but after the cats(if you still have them). i would just do a H-pipe and around 4-6 inches after they come together on the same side. for the cross over pipe in the H, you want it to be a size smaller. i.e. you use 2.5" duels the cross over pipe part should be 2.25". its meant to keep the velocity up between the 2 banks. if its 2.25" duels you can stay with a 2.25" cross over from what i hear.