Seal V-Band with RTV...have the race pipe section and the u-bend delete section that bolts to the catback welded up...I also recommend getting rid of the flange from the catback to the backhalf of the downpipe...that way that is all one piece...you're eliminating three possible leak spots, and can drop the catback with just the v-band clamp.

Not that those will affect fuel trims because they are post o2 sensor...so get the leak on the crossover or the rear header fixed first.