Large flat head screwdriver in the clamp wedged around the edges to loosen it up, then I grabbed the corner of the cat pipe and push pull back and forth... With the hanger at the corner off you can even tap on the cat pipe at the elbow staight up and down to break the seal between the cat pipe and the header flange.. That V band clamp really presses the cat pipe onto the header so it can be "stuck" a little bit even after the clamp is loose.