Somelee, mine were installed by the GT Guys at the Rally. We looked at all the options but drilling new holes and came up with a combo of Black RTV silicone and the use of the original fender "buttons". I drove the car the next day at the rally through the mountains at a "good" pace and they worked perfect.
The only problem with the guards is they are very low and after lowering my car a little over an inch they (the splash guards) scrape on speed bumps. The cars clears the speed bumps fine.
I have also noticed that they do work quite well. After following a few other GTs through wet areas my GT still looked freshly washed, free of dirt/mud/debris and the other GTs did not.
If you plan on putting a lot of miles on your GT these guards will go a long way in saving your paint and keeping your car clean.