Well, let's see the down pipe is 2.5 until the split/tranny oil heater pipe, then it's 2 and a 1/4
What I plan on doing is chopping the DP right before the split adding a 2.5 90 bend to a flex coupler then v-band>to v-band to 2.5 cat. then straight pipe to a round magnaflow then pipe to magnaflow muff.