wow....
Please do this before you waste anymore money...
Buy a 94/95 GT X or H pipe with the aftermarket cats, Bassani offers a few nice items. I say 94/95 because the GT 5spd used a T5 just like the six and it has the same trans mount location. I am not sure about the modular versions.
Take that to a muffler shop and ask them to fit it to your stock manifolds. Tell them that they will need to change the flange size and angle. Mine cost $70 to do.
While you are at this you may as well upgrade to a set of shorty headers, the 94-97 work well (98 if you block off the air injection.)
After you do that go out and buy a side exhaust kit for the GT like a Roush or something.
Save you a lot of time and trouble and having the GT's true dual exhaust you can swap to any GT muffler/tip combo.
You can see how it looks under my 96' here, and listen if you want to but the sound quality sucks.
www.YouTube.com/CrainRacing
Ignore the stock muffler hanging there, I took the vid first before I removed it.
Good luck and just get it done right, please
-Eric-