The Reese website has been updated and shows both the classic (Select) series 22k 5er hitch and the Signature Series 24k 5er hitch are available for the 2008 F-450.
Based on Reese part numbers, any hitch for a 2005-up SuperDuty pickup should work on your F-450 pickup. So if you want some brand other than Reese/Drawtite, and if they don't list the 2008 F-450, then just get one for an '05-up F-350.
If I had a 2008 and needed a 22k 5er hitch, I'd get a Reese/Drawtite install kit for their Select Series (original or classic) 22k hitch, i. Be sure to get the Custom Quick Install Brackets.
Surf to www.reeseprod.com
- then click on fifth wheel, then on "Rails and Bracket Kits", then on "Custom Quick Install Brackets". Note that the 2008 F-450 is listed as one of the vehicles those brackets will fit. Those brackets are also called "Hidden Hitch" brackets. They make installation of a Reese 5er hitch - or any brand that uses Reese-style bedrails - easy as duck soup.
Dick Cutting (Dieselstop moderator) has those brackets on his F-350. He raves about them.
If you literally hate bedrails, or if 22k is not enough hitch, then Reese also makes a 24k Signature Series hitch. They use a different mounting kit without bedrails, and it shows that they have a mounting kit for a 2008 F-450. Instead of bedrails, 4 little titties stick up into the bed.
Reese also makes a 30k 5er hitch, but it is bolted directly to the frame of a narrow-frame chassis cab. I don't think it will fit a pickup without some serious fabrication of a big thick steel plate under the bed.
You can order a Reese/Drawtite/Hidden Hitch 5er hitchs from most RV stores and trailer hitch stores. You might save a few bucks by ordering it from the internet, such as
= 22k classic series
= 24k signature series.
Be sure to order the correct install kit for whichever hitch you decide on.