I'm pretty much set on the Magnaflow muffler 5x8x18 (122678) because of the sound level in the cab (or lack of). I'm gonna let a muffler shop do the pipes. Now for the question, I really want to dump out the (pass) side in front of the rear tire. I DO NOT want cab drone though. All the magnaflow I've heard has been rear exit. How much more noise should I expect in the cab from this setup?

 

Go with the side exits... i just had them installed on mine and i love it. There is no cab drone on the highway/roads. But you can hear it when you punch it. I have the exact setup your thinking of 18inch SS SI/DO 2.5 inch muffler. You wont regret it