Here's what I'm planning on building for the 3.5L V6 in my Nissan 350Z:

30 to 32" primaries (don't know the diameter yet) grouped 3 into 2. The first collector will have a 15 degree merge to a 2" outlet. Secondary pipes will be 6 to 8" to the final merge collector with 15 degree merge, 2.5" outlet, 7 degree cone to a 3" final end. 3" exhaust all the way out the back with a Borla XR-1 Raceline muffler and room to mount a 3" SuperTrapp for the tracks with noise restrictions.