Magnaflow makes nice low restriction cats and straight thru mufflers. Look for total stainless construction, screen over the perf holes in the tube of the muffler, and mineral wool packing. I seem to remember that the Magnaflow muffler flows better than a straight pipe of the same length. I am a believer in the theory that unless you change the harmonic properties of the exhaust (i.e., pipe length), diameter will not affect low end much at all. I think the 3.0 stock cam has minimal overlap, which will make the effect even less.

Not responsible for advice not taken...