I'm wanting to do a semi-custom exhaust and I'm not for certain on where all the pipes line up and what will or will not fit.

Here's what I'm thinking, still trying to figure out if I can get the flanges and gaskets needed. Rotary-works 2.5" header -> dual 2.5" universal presilencers -> N1 dual cat back minus the little "y" section to keep it true dual.

I'm trying to contact rotary-works on seeing if I can get another flange like the rear one for the header, which is SS304 and a gasket. I'm going to be doing all the little piping needs in SS304 as well and TIGing everything together. My main concerns are length between the header and N1 and also if there is enough room for dual pre's under the car.

Here are pics and links to everything.
Header
N1 Dual
2.5" Universal Presilencer

Edit: Also, if this all did fit what kind of sound levels am I looking at?

 

Okay, so going off the N1 picture and RacingBeat's exhaust hanger chart for approximations it looks like I will have room, length-wise, since the N1 duals start where the stock "y" starts spliting if I ditch the N1's little "y" piece.

Now, I just need to figure out if 2 of the presilencers will fit side-by-side in the tunnel or if I am going to have to offset them, assuming there's enough length for that. Also, sound level is my final concern.

 

if you look closely you will see that the pipes already have presilencers on them.
http://www.rx7store.net/product_p/163-kz01.htm

 

I did notice that, but I wasn't for sure that it would keep things quiet. If only RB made a 2.5" version of their RR presilencer with a 6pi tube lmao.