Keep the standard manifold, downpipe and cat then go 2inch from there back. Unless your fitting a 4branch but u still need a box where the cat is. Its important where the first box in the system is, its a tuned length because thats where the car thinks the exhaust stops in terms of pulse and velocity. A 4efe with a peak power at 4500-6500rpm is best fitted withthe 5efhe manifold and downpipe and the cat in roughly standard postion. The cat can be gutted out.. Search four stroke exhaust length calculator.