Here's what the heck I would do, is run dual 2.5 off your manifolds into a nice Y to 3" out then into a 3" walker quiet flow, over the axle straight to the bumper. I have my 00 f150 5.4 like this and it sounds smooth and mellow. The second half the system being a single pipe smooths out the exhaust fluctuation.
here you can get a proper flowing Y pipe. You can get a (2) 2.5 in and (1) 3" out.
