Per Trailer Life - 2008 Towing guide:
http://www.trailerlife.com/images/di...wGuide0801.pdf
The 2008 F-450 is rated for 5th wheel towing as follows:
Ford F-450 Super Duty, Fifth-Wheel Towing
Vehicle Engine Tow Limit (LB)
F-450 Crew Cab 2WD 6.4L TD 20,500 (a,j)
F-450 Crew Cab 2WD 6.4L TD 24,500 (a,n)
F-450 Crew Cab 4WD 6.4L TD 20,000 (a,j)
F-450 Crew Cab 4WD 6.4L TD 24,000 (a,n)
F-450 Crew Cab 2WD 6.4L TD 18,500 (m,j)
F-450 Crew Cab 2WD 6.4L TD 19,500 (m,n)
F-450 Crew Cab 4WD 6.4L TD 18,000 (m,j)
F-450 Crew Cab 4WD 6.4L TD 19,000 (m,n)
So think you have plenty of room for that Designer. Go to page 27 and zero in on exactly the 450 you are looking at.