This is MY OPINION, so take it for what it is worth.
Custom built 2.5" y pipe into a single 3" IN/OUT high flow cat (Thunderbolt or Magnaflow) then 3" pipe into a CB extreme or 14" magnaflow and then ran all the way past the axle to the exit of your choice...
I have just single 3" pipe from my muffler out the back tucked tight to the bumper.