I've towed lots of crap with my gtp (2k miles or so) and It does real well. I'm not concerned with my trans at all since I can rebuild these things no problem and modify them a bit. I've cracked the trans open twice in the last 3 years and my frictions were fine from towing.
Things to do when towing
Don't tow in overdrive since theirs only 2 frictions in the 4th clutch pack as opposed to 10 single sided frictions in 3rd. Leave it in 3rd and just keep a nice steady pace of 65 mph or 70 mph and you'll be fine. Definitely leave the supercharger belt on as the car will be pig slow (dangerous) without it. I've towed with my gtp with the stock gears and with 3.29 gt gears and they both pull great. Be aware when you go over railroad tracks especially when your lowered. My car has a gmpp springs which drops the car a bit so I always slow down over the tracks when towing.
You'll probably only get around 18-20 mpg when towing for a heads up. It's a good idea to keep an eye on trans temps if you have a way to monitor it. If not, just don't be flooring the car and be easy on it and you should be fine. I recommend you buying a Uhaul class 2 stealth hitch. This is a great hitch and is currently what I run.
Look at this other link I responded too
http://www.grandprixforums.net/f59/tow-hitch-10017.html
