The GT cat back should bolt up to the h pipe, and have the mufflers welded in, Then just have the tips welded/ clamped on...I think i remember Paul (greenflash) running into a problem with that same setup though, may different for a 94-98?
Well if you want a good price there is to ways to get the most bang for your buck on a new exhaust system.
1. Get a great deal on a exhaust kit and go get it welded to make it more secure.
2. Go find a local muffer shop preferably a privately owned shop and have them custom make one. Here where I lived I called every shop around in a 15 mile radius. I got quotes as high as $550 bucks and as low as $200. If they tell you they need to see the car blow them off. Any shop with experience knows what it takes to install a system on a Mustang. If they need to see the car they dont know their business and be very afraid and keep away from them.
I got my custom system including Dynomax Ultraflows for $ 250 just by shopping around letting my fingers do the walking in the yellow pages.
If you live in a city with little choices or you live in some stuck up, pricey, northeastern or Pacific Nothwest I am sorry and you might need to get a kit. Some people have reported that in these areas there are no good deals. But check for yourselves.