[Lona]

Colonel,

The rear axle is a little too far forward to attach belts to and there are no structural rails aft of the axle. I installed a 1 1/2 x 1 1/2 steel angle under the rumble floor pan at the break where it angles upward at the rear but made it the full width of the space between the main frame side rails. The steel angle is drilled for the bolts that hold the seat belt brackets. Lots of room there to work so it's a fairly easy installation.
As for the rail cushions, I just slit a standard kid's swimming noodle and slip it over the rain rail whenever kids sit in the rumble area. Could also miter cut the ends and install the same cushions on the side as well. All easily installed and removed as the lid could not be closed with the cushions in place. Good luck with your install.

Glen