Have a custom carbon fiber driveshaft made for your car would be my recommendation. You'll definitely notice a difference. They're not cheap, but they're worth the money. The one I had on a car in years past was made by ACPT. http://www.acpt.com/
One thing's for sure, when you receive the box, you'll think they forgot to send you the driveshaft it's that light! The box and the packing material weighed more than the driveshaft!
I just did some searching for you and PST makes carbon fiber driveshafts as well and it looks like they already have the specs for the GT500 so you could check them out too. Either company would be fine in my book. http://www.pstds.com/custom_driveshafts.htm