i think youll have a hard time replacing the first cat with a highflow cat...i think you dont know where exactly or what exactly your first cat looks like...first off, its a triangle setup (3 bolts) custom looking thing...second, it looks NOTHING like your second cat, rather its a big cylinder looking thing...maybe they make highflow cats for applications like that, but ive never seen one, and i spent quite a bit of time looking.if anything you would remove your first and high-flow cat into the second...if it was easy to replace the stock first cat with a hi-flo one, then these other header manufacturers that allow you to keep your stock cat would just replace the stock cat with a high-flow one.