5th wheel installation

I got a 5th wheel set up from a local auction. It has the plate on a crossbar, 2 large u-shaped hoops that go from the crossbar down to the bed, and 2 rails that would then go across the bed. My question is, how does this install to the bed?It doesn't seem like bolting the rais to the bed, not the frame, would be a good idea. I can't imagine it supporting the weight even if I bolted it thru the bed support crossrails.

Should there be supports across the bed underneath, then drill and bolt? Do the rails goe under the bed and then drill holes for the hoop legs to go thru? If it should have support, what size? 2x2 box bolted to the frame?

Also, should it be centered over the axle, or slightly forward?

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