Eddie I've been looking into the same thing. So far Kelderman, Fabtech, and Link UltraRide are the only off the shelf systems I've found.
Earthroamer uses a 4" Kelderman air suspension system. Specifically the 2-bag front, 4-link rear, and 4-way self leveling air control system. With this system they remove the rear springs and use a 4-bag system. Kelderman also offer a 2-bag rear where the rear spring shackle is disconnected from the frame and attached to the air bags. They also offer several levels of air control systems.
http://www.keldermanairride.com/medium-duty/ When I contacted Kelderman they indicated that the F550 4-link setup was considered a custom system and they would prefer that the install be done by them in Iowa.
Fabtech advertises several 6" lifts for the F450/F550, including a Basic kit, Radius Arm System, Radius Arm Coilover Conversion, and 4 Link System. Each is offered with or without their upgraded Dirt Logic shocks.
http://www.fabtechmotorsports.com/applications/ford/4wd08ford450.html
When I spoke with them last week they indicated that the coilover conversion wasn't yet "approved" by them as it requires some modification of parts during the install. Not sure why it's offered but not "approved".
I haven't researched the Link system yet. As Charlie mentioned the other option is to home grow a solution. If you take the plunge let me know.
Bob