A longer exhaust builds torque and the last thing you want is your exhaust barking from under the cab. It will become fatiguing in no time. And it'll piss off the cops if you ever get pulled over for a 'routine traffic stop'. If you know your stuff you can actually length tune the system but it's really a specialised technique as part of a full engine tune set up. We have laws here in Oz that the exit from an exhaust must be from behind the rear axle and must not be closer than 1 metre from the nearest opening window for passenger vehicles.