Mounting a fifth wheel hitch?

I recently traded my 2011 F350 dually for a 2016 F350 dually.The 2011 had a Husky hitch with Reese custom frame brackets. I went to a Reese dealer to be sure the 2011 brackets would fit my 2016 frame. He told me they never use any brackets to mount the rails.According to him, they use the 4 center bolts holding the bed to the frame. The rails have a round hole in the top at each end. I did some measuring and found that the bed mounting locations do put the rails in the correct position. He said to use grade 8 bolts. My question is, has anyone ever heard of this? I am not sure the hat rails on the bed would stand up to the strain of a 16,000 lb. fifth wheel trailer pulling and pounding on them.These rails between the bed and the frame are approx 1.5"X 1.5" and made of the same metal as the bed floor. It would be a very easy way to mount the fifth wheel rails but I am just not sure of it's strength and durability.