4in straight pipe is to small ,5in straight to a 8in tip. Oh you also gonna need some more air coming in. Any CAI will do. The 6.7 is a beast but the government has it on a leash in stock form, you gotta unleash it to see the real potential.
Well if it was my truck first thing i would do is get rid of that DPF. They make dpf delete kits. The dpf traps all the soot that gets emitted from the motor and once it gets clogged its expensive to get cleaned out. The more air coming in and the more air exiting out makes a happy diesel. In my previous truck (06 3500 5.9) I ran a 5in farm boys diesel straight exhaust with a 7in tip, Fass 2.0 fuel system, 6in CAI (homebuilt with, amsoil cone) and i used the smarty for engine managment. Oh make sure you get guages if you plan on modding the engine i cant stress this enough. I ran 37-13.5x17 on my dually and estimated 460 RWHP. Just like jeeps diesels are a money pit and the power can become addicting.(edit: if you follow my mod list beware my transmission went by-by in less than 5 months, it was a auto)That looks sweet with the tires {toast}