I love the idea of a flat bed by using a gooseneck hitch and some kind of fifth wheel adaptor. There are several ways of doing this but right now the one that really catches my fancy is the Curt X5 Gooseneck rails adaptor.
Basically, if you have a B&W or a Curt Gooseneck hitch then you can drop this adaptor into the gooseneck receiver and you get a set of standard rails. After that you can add the fifth wheel of your choice.
Here is an example on
etrailer for the B&W gooseneck.
Its pricy but it lets you use any fiver hitch and leaves your bed flat when removed.
Me personally, I have never had a fiver but my dad and best friend have and they both have the permanent rails in the bed. We have never had a problem with them being in the bed. Being honest with ourselves... the flat bed thing is for aesthetics for the most part. You can cut a drop in bedliner out for the rails and pretty much have a flat bed. You can do the same thing with one of those heavy rubber mats that people put in the bed floor of their trucks.
If you go with permanent bed rails installed and you need to haul a gooseneck trailer you can just get a
bent plate gooseneck ball that drops into the fifth wheel rails.
Thanks!
Jeremiah