I have a set of 1 7/8" to 2" Kooks stepped headers I would like to install soon and I am tring to iron out all the details on the new exhaust and procure the materials. The headers have 3.5" collectors and I was planning to run a 4" single exhaust out the rear pass corner. I will be building the exhaust myself with prebent pieces and do not have access to a bender so accessibility and routing will be part of the challange. Since the collectors are 3.5" I am considering a 3.5" into 4" y-pipe. I am thinking this may be overkill for my setup and a bit more difficult to fabricate. I am also considering using reducers on the collectors and just building a 3" into 4" ypipe. The plan is to support ~750 whp and I think the dual 3" will do that plus give a little more velocity for better off-boost response. I just wanted to get some opinions here and see if anyone has been sucessfull in fiting 3.5" pipe off the headers in a NBS truck and their performance results with it.

 

Bueller... Bueller...??

Here is a quick sketch to show what I am thinking...

 

im running a 3" y-pipe to single 3.5" exhaust and its a pretty tight fit but then again my truck is AWD so you should have a little more room than i do. I dont think a 3.5" y and single 4" will be overkill for your setup. id say go for it!