Have a look at these threads.
http://ecomodder.com/forum/showthrea...vic-16422.html This one's my own belly pan build, with some notes on how I dealt with the exhaust system.
http://ecomodder.com/forum/showthrea...how-15026.html
There's a ton of aero mod threads in the Project Library in the section "Project Section: Aerodynamic mods - fabrication", and more than a few discuss belly pans:
EcoModder Project Library - EcoModder.com