Checking out the LPP headers, seem like a sweet deal and just what I want. Only problem is they are 2 1/2 inch y pipe, not sure what the stock size is but
I know stock exhaust is 3.5. I am leaving the stock exhaust that I cut the resonator off and I am just going to run a cut out before the muffler. So my question is will the 2 1/2 inch y pipe hurt me?

 

Just have them Y the 2.5 duals into your single 3.5 pipe.

If you ran dual 2.5 pipe it will flow more than your single 3.5.
Single 3.5 = 11in sq
Single 2.5 = 7.85in sq
Dual 2.5 = 15.7in sq