you want to be careful modifying the exhaust with the stock carb without the cat - it will definitely uncork the motor to the point of leaning it out badly. I would do a cat back with a single 2.25" inlet / dual 2"outlet with a cat in the system until you put the weber and header on. I ran Geronimo without a cat with the weber and a header for years, but had to put the cat back when I put a new exhaust system on, and he runs better - its a free flow 2.25" cat for a 5.0 mustang, so the restriction is minimal. Geronimo is a 2.6 with a 5spd.