An x pipe will net you a higher peak hp. An H pipe will give you more down low torque. I would use the H pipe considering your car appears to be stock, and will probably not see much time over 5,000 RPM. Does your motor have headers on it or manifolds? If you have the manifolds I would probably run a 2 1/4" dual system, headers a 2 1/2". Do you plan to dump the exhaust at the rear, or go all the way out the back? A dumped exhaust has ALWAYS sounded better to me. Tailpipes usually distort the sound of the muffler and give it a semi splatty sound under part throttle. I put the new flowmaster super 40s on my 340 'Cuda and couldn't be happier with the sound. It is very deep, and sounds amazing going down the road. A regular 40 series muffler can not compare to these. I had glasspacks on my D300 at one time, but over time they just burnt out and it was incredibly loud and raspy inside the truck. My advice would be to get a fully welded muffler (stainless if possible) for a long life. Hope this helps.
