i would start off the cat, split it, run two mufflers, i recommend moroso spiral flow mufflers or flowmaster original 40's, then over the axle and either behind each rear tire, or under the bumper on either side. this would be the easiest and sound the best. you could do two pipes off each manifold, dual 02 sensors, duals cats on so on, but the cost does not pay. plus theres no performance gains doing it that way.
oh and def do 2 1/4 pipes, biggest 2 1/2 dont go any bigger.