Ahhhh ok, well in that case, I have no idea sorry.
With a few of my cars, what I did was go between .5" and 1" bigger diameter from the manifold to the rear muffler, and then drop the tail pipe back .5", but make it twin. I found this didn't give my cars that shitty "I've welded a 12" tip so my car sounds like a jet" ricey sound. Also I am not one for loud cars, the cops are very strict over here with the volume that comes out of the exhausts, so I tend to go for a nice note instead.
Your best bet is probably to ring a few of your local exhaust shops and get their opinions on what would make it sound/perform better, and then make a descision from there. I've found you can often get a good custom made, mandrel bend, exhaust system cheaper this way than going for one of the big named pre made ones.
Jay