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.
There's always kits on ebay, but if I were you I'd skip em and stay with tailpipes.
I have the SVIO skirts and magnaflow muffs, and while it sounds great and looks great it does everything the previous poster said, PLUS you cant have any conventional subframe connectors with them.
I have a panhard bar, so I cant do a rear-exit, so I'm gonna end up having to custom fab new subframes or run dumps, which sucks