Details for fastening folding truck step to outside of 97 Tacoma bed?

I'm looking for construction details and advice on fastening heavy duty folding truck steps to the exterior walls of my 6' bed. As a contractor, I do a lot of tying down of loads and want something more than the top of the rear wheel and bumper to stand up high.
Brackets fasten with two 5/16" bolts:

I want advice on the best means of fastening to the sheet metal.
Since there is a bed liner, I need a diagram showing where reinforcement is located in this presumably double wall construction. I could conceivably use short screws into just the outer layer but I'm concerned about my weight (200 lbs) distorting the panel.
Should I use through bolts? Are there special hollow wall fasteners, something like a molly bolt that would be sufficient? Should I consider mounting the brackets onto a larger base plate and distributing the load with 4 or more screws instead of two fasteners?
I tried looking for a cross section diagram, but the best I could come up with is this:


Any help appreciated

 

@Poindextrous Did anyone get back to you on this? I have a similar idea, possibly mounting a folding step on the upper tailgate, but wasn't sure if anyone had tried it. Thanks.