Cervini Type III is your best bet if you want to have a direct fit unit designed for the GT front end. The Astra Hammond comes a close second but both kits will be around 200 bucks vs less then 100 of a hacked up GT500 unit if you can do the handwork needed to make it fit.
Personally, I don't like things that aren't made for our car. The GT500 spoiler looks ok, but it doesn't fit quite right, and there's a gap in the middle, unless you put something in there to block it.