There's a side-exit skirt/muffler/pipe cat-back kit available for '94-'98 Mustangs from (I think) Dynomax, but...
1. It's expensive, for what it is;
2. It doesn't sound all that whooptie-doo;
3. You've gotta factor in the extra cost of getting the skirts painted;
4. You will loathe the day that you go a bit too quick over a speed bump that's just a hair too tall and you wind up cracking/breaking one or both of those skirts;
5. If you have the windows down and you're sitting at a stoplight, I hope you enjoy the smell of exhaust.
I used to be hardcore about side-exits until I bolted up three different setups on two different cars, always with the same above results. From now on, it's tail straight out the back for me. Just my experience with 'em.