It was designed with the intent of supporting a RTT and the occupants. I don't see it being that different than one on a roof from a mounting perspective.
Sent from my XT1585 using Tapatalk
It's actually a fairly big difference. So the roof has cross members and trusses that run across it for structural support. Also the trim of the roof, where one would mount brackets for a rack, is further beefed up, in turn creating a very stable platform for mounting.
Conversely, the cargo rail in the bed is not designed for safty in the event of rollover so it is only secured to the sheet metal with a few screws and plastic anchors. The rack will help distribute weight across the bed area but with only a few fastening points on the rail, the load would still be transferred to those points.
If there was a way to reinforce those points, which I think there is if you remove the ram boxes, the rail could be made to support more weight by adding a thick piece of stock steel in parallel with the rail and spreading the tension load of the few small screws across the entire length of the bed sheet metal.