Here is something new, I built a 2.5 stainless exhaust from the manifolds back with a 4" mid-pipe that has a single 4" Magnaflow muffler followed by a pair of 2.5 Flowmaster Delta 40's in the stock location. Also used a pair of 3" cats to make it "clean".
Here is a video link to see/hear it. Hope to have a driving version soon. It sounds great and very quiet for Delta 40 Flowmasters. The 4" mid-pipe with the Magnaflow muffler has a slightly larger sq in area that seems to slow down the air some to give it a deep sound without the dreded 40 Series drone. The Magnaflow coupled with the 4" mid-pipe really quiets it down alot and gets rid of that poppy sound under heavy load and deceleration.
Don't mind the smoke, the MAF was way rich with the new exhaust plus the j-bends I used to make the exhaust were soaked with oil inside.
Even though it still breaths thru manifolds, it made a big difference for power. It would be nice to see dyno numbers, but the track should be able to show the pickup. I built it with flanges in stock locations in order to add headers later or change the axle-back setup.