Wow! I thought my buddy and I were but a few using the Shuttleworth product. Paul Fox @ Orange Muffler spliced a Shuttleworth into my '07 5.9 over six yrs ago. Throatier sound with no drone. Mine is not the straight through one. Minimal corrosion after living 4 miles from the ocean too.http://www.shuttleworthexhaustsystems.com/
I've got a 5" Shuttleworth and love the tone. I have no resonator and have no drone while towing. It is a straight through type as well so no flow restrictions to worry about.
I don't know of any that have the offset of the OEM one, but you get a muffler elimination pipe and then put a different muffler closer to the cab.
The Donaldson 40" and 51" mufflers are popular, cheap, and sound great. The 40" is a little loud with an exhaust brake, but otherwise I liked it by itself.
40" M085171
51" M090544
Try a Nelson 86127M 40" OAL x8.5" OD or 86131M 51" OAL x 9" OD. David
Walker 21471