Lets make this easy. Sd headers come with a straight downpipe no cat that bolts to the stock resonator assembly.
See the 3 pipes on the bottom? The straight pipe, the flanged pipe and the one with the bend in it? Those are 3 inch and are the downpipe. They clamp together and clamp from the flex on the rear header to the flange on the res assembly.
Now the straight part of this can be replaced with a 3 inch cat. Your best bet is to get it welded in. Just go to your local auto parts store and ask them for a 3 inch cat that would be cal compliant.
