I used a 2-into-1 collector under the front passenger's feet, and ran it into a single 2.5" high-flow cat. I welded the connection at the cat and drilled through the 2 layers of pipe on the inlet side of the cat to weld the bung in for the O2 (gave me more material to keep from blowing holes in the pipe with the welder). My exhaust is 2.5" stainless from the collector back. Flowmaster collector, Magnaflow cat and muffler, mandrel bends and straight pieces welded together for the rest.