I'm sure you've seen the directions on how to modify the gt500 splitter to fit the standard gt bumper... well, this is very similar, except that I removed the entire raised 'grid' along the backside of the splitter, including the center section that gt uses; didn't need it because I'd made my own piece to fill in the center gap.

i removed the front plastic cover from underneath the bumper and stored it away. i suppose it could be reinstalled, but I didn't bother. to make the center piece, I just measured the opening in the bumper, cut a piece of plywood, trimmed to fit, and applied an adhesive backed flat stock foam rubber trim to the front edge of the plywood. I also used 3m double-sided tape better adhesion of the foam rubber. Once that was completed, I drilled 4 holes and used (I think) 1/4" bolts, along with wide circumference washers (washers over and under) and vinyl lock-nuts to secure them.

once the center was installed, I lined up the splitter and used the same method to secure it to the underside of the bumper; about 8 bolts total and that thing is on there for good. the sides are secured the same way as the gt writeup suggests.

this is the best i could do for pics, as didn't take any during the installation...

i only used one screw at the top for each side. that seemed to be all that was needed, so i left it at that. all of the original wheel well/bumper screws remain in place and i drilled holes for these...





here you can see how the foam rubber creates a nice seal as it's sandwiched between the splitter and the bottom of the bumper...



btw, when you install the center piece, make sure you offset it about 1/2" - 3/4" behind the leading edge of the bumper. when i first installed it, I mounted it right along the leading edge of the bumper and it didn't look right, so i moved it back a bit and achieved a much cleaner look.

some under-body shots (driver side, pass side and a bit of the center)...







as you can see, it looks quite rough from underneath, but it's not a show car and it sits so low that there's not too many people, besides myself, that are ever gonna see under there, so it's all good afaic. I'm sure that there are a number of other materials that can be used, as opposed to plywood, which would provide a cleaner look; i may look into them if the wood ever starts warping (lol).

hope this helps!

Paul