its all to do with the backbox and resonating chamber gubbins inside the thing. the 355 replica is a single transversley mounted backbox, the pipe from the manifold goes over the box and enters the sides from the back end of the side pannels, (you follow??) then two pipes each side come from the leading edge of the box and then head out towards the rear end of the car.
355 replica - its by the wooden shelves!!!!! :-
Compare that to mine with two sperate boxes and two feck off huge pies, it creates a compleatly different sound, both sound good though!!!
Mine:-
HTH
Matt