I've got a 3" DTS down pipe to a resonated Diamond Eye front pipe thats 4" back to a 6" stack. see my thread with my truck pics to see the stack. It's a black MBRP i had blasted and powder coated Black Chrome along with the Diamond EYE billet clamp.
I personally have a 5 to 7. I got the stack from a semi shop. The 5-7 adapter is killer $$$ for a Chrome one. Mine was for performance more than for looks.