From a show I saw each cat has a different job to do so eliminating one might not work. Also separating the two might have an effect on their running temperature and screw up the whole process. I thought of going through the frame as well. Same as posted using a pipe welded in but also adding a gussett to both sides that is welded to the pipe as well as the frame. This same sort of thing was used to extend a truck frame for a ramp truck conversion. I wouldn't attempt it without the gussetts.