If you like the straight pipe look freedog...I think Kenworth has the same "retro" exhaust that Peterbilt came out with (I have it)
They just "look" like straight pipes...but there's really just one bigassed muffler under the truck, tucked up between the frame rails. It is also extremely quiet, & I also noticed it's way easier to back into tight spaces cause you don't have that damned muffler to look around!
You may be able to retro-fit it. But whatever you do...don't just buy the "toppers"...they look cheap & fake!