Here is what I did on my '98 Dutch Star w/300 Cummins. Replaced the factory muffler with an Aero Turbine XL muffler. It is all SS and straight through design. Google searched and found one w/ a ss tip for $125 & free shipping. Mine was a simple install, took maybe 2 hours.
I don't have exact numbers but probably a 1/2 mpg increase. Better than that, it will climb mountain passes without as much overheating.
(went from Texas to the salt flats & back in August pulling 12K trailer) Also not much louder, if any.
Looks like you will need a little custom bend but should be fairly simple.
Rafe
