964 exhaust is very different from 993

here's yours stock
http://forums.pelicanparts.com/uploa...1206112194.jpg

For your app many use a cat bypass an remove the final muffler(#9), the intermediate muffler out can be modified to go out the stock opening.

another more expensive option is the Fabspeed header/exhaust which works quite well.
http://forums.pelicanparts.com/uploa...1264459998.jpg

I might look into this for starters.

I have the first muffler gutted on my 89. It isn't much louder unless you are really into it. I do have some resonance at a higher rpm, can't remember where. I would probably swap both mufflers out for one of those cat back systems from GHL, Fabspeed. You can get the left side valance piece here for dual outlets: FVD Brombacher Online Shop