Cat back....

just get an o/r H or X pipe and have a muffler shop weld up some flowtubes to a set of magnaflows if that's what you want. it'll sound and run A LOT better to have a true 2.5" offroad pipe instead of a gutted, stock, 2.25" pipe. o/r midpipes are so cheap it's almost rediculous to settle with some cheap, hackjob stock pipe.

If you want a muffler like a Magnaflow, go on Pypes website and look at the Race Pro catback. Part number #SFM10R for a 87-93 GT, you can call and have American Muscle order it for you, 239.99 shipped to your door.

NAPA used to carry a 24" section of 2.5" pipe that had the flange and bracket to bolt to the H-pipe, it was less than $20 a couple years ago when I bought a set.

If i'm not misunderstanding you, you looking for "flow tubes." They connect the H-pipe/X-pipe to the mufflers.

I think this is what your looking for.....
http://www.drgas.com/store/product.php?productid=8&cat=1&page=1

It'll connect, but as it's been stated above, gutted stock cats will flow like crap. If you're needing to pass visual, your best bet is to look for a cat'ed high-flow H- or X-pipe. A CatCo H-pipe with high-flow cats can be had from Summit Racing for right around (or just under) $300.

DO NOT bother looking at the UPR cat'ed X-pipe, or any other designed similarly - I've got one, and it's utter garbage. (Doesn't fit worth crap. Cats hit the tranny crossmember, front-end hangs too low, keeps sagging at the rear flange.)

Alternatively, you could always score a used off-road H-pipe for dirt cheap (I've seen 'em go for $40 to $50 all the time, and they're only around $100 brand new, anyway) and just swap your stock H-pipe back on there whenever it comes around due for inspection.

You *may* be able to get away with buying a couple of high-flow cats and an off-road X- or H-pipe and welding/clamping on the cats, yourself, to save some money, but you might run into fitment issues with that kind of a deal unless you're reeeeeeeally good at measuring things out in advance.

After that ... as suggested, just buy a pair of flow tubes ($40 a pair most places), bolt those on, and slip on whatever mufflers you want behind that. Make sure to clamp on a couple of hangers to the muffs or turndowns if you plan on dumping it ahead of the axle.

EDIT: If you're worried about passing visual inspection, I'm pretty sure that most places don't allow dumps, anyways, so you may as well shop for an actual cat-back. I second the recommendation for a Pypes cat-back - I'm very partial to the Violator mufflers, myself.

Ya know, if there's a recycling place around you that takes scrap metal, there's a good chance that they'll pay you cash for your converters....pretty decent cash (if it's a stock 4 cat then probably 80-100 bucks depending on how full the cats are). It's possible you could sell it and get enough cash together to buy an offroad version from the classifieds or craigslist or something like that.