Well I have the headers and valved GT3 muffler set up. Now I just need to have the rear bumper modified to accommodate the center pipes and close off the stock passenger side exit.

Can anyone recommend how to close off the passenger side? I was told by a body shop that the hole is too big for plastic welding.

 

The body shop I used bridged the gap with wire mesh and I believe filled the gap with a plastic welded piece from my old bumper centre section.
A light skim of filler and then make it look OEM.
Been like this for nearly 5 years now and no issues...

 

Yep you can plastic weld a big hole if you have a filler piece. The entire front license plate section of my front bumper was cut out, heated and bent, and then plastic welded back in place so that it's totally smooth.