Installing 5th wheel on custom flatbed

[Jason]

I have a standard rail type 5th wheel hitch for my f250. I have removed the rusted out and falling apart bed, replacing it with a flatbed I built. The rails are designed to be bolted to the frame brackets through the floor of the bed. I'm sure the bed would have been capable of supporting some weight on the rails. I bolted treated 4x4 lumber on spacers to each side of the frame and screwed 2x8s perpendicular to the frame. My intent was to be able to remove 2 or 3 of the 2x8s when I need to install the hitch, and have a full flatbed the rest of the time (living in trailer, moving 2-3 times per year). Without the bed there, the rails only sit on the frame. Do they need the support the stock bed would/could have provided, or can I just bolt the rails directly to the brackets?

Thanks,
Jason