You could put a Borla XR-1 #40086 (5"dia., 3.5" in/out, 15" case length)muffler in the I-pipe, but you'll need to clearance the drive shaft tunnel a little bit on the passenger side so the muffler will tuck up better, it's about $125. Or use a Moroso Spiral Flow(4"dia., 3.5"in/out, 12" case length) muffler in the I-pipe with no need to do any clearancing of the tunnel, about $55 steel(#94052), or $102 in stainless steel(#94056).Also a Dynomax #24216 (same size as the Moroso) about $48.
The dynomax and borla shouldn't affect the flow at all , the Spiral flow will effect it a little bit.

 

Thanks for the info, do you think it needs quieting? I mean for a neighborhood?

 

Well, that's something only you can really decide.

I don't think any of the options I presented will make it quiet, just tone it down a bit.

 

http://www.summitracing.com/parts/MPE-14909/

would this muffler fit? Its much bigger than the one already on there

 

That will work, with a little bit of pipe trimming to get everything lined back up.

 

Sounds good. More aggressive would be great though