What kind of stuff should I expect to happen should I want the exhaust to come out the sides just before the rear wheel? Currently, I have some dual 40 series Flow's on it. Here's a short list of questions...

What effect does it have on performance/sound?
Can I just use existing stuff and just shorten everything?
What is the emissions legality of it all?

I have done just what you are thinking about. It will have practically no affect on performance except since it will be louder inside the car you think you're going faster. It will have no affect on exhaust emmisions but it may not be legal to end the exhaust pipe before the rear axle--check your local laws. You should be able to modify your present exhaust system but whatever the modifications, it is easy.

It is deffinenty going to be louder inside the car if you want that. On one of my Chevelle's years ago. I had headers, header mufflers which I stuck pipes bent at a 45 with chrome tips at the rear tires. I liked the sound. That was when I was young & liked way loud!