Let me start by venting this is the second time I get to type all this below, damn internet cut out as I hit submit reply, which in turn deleted everthing
Anyways, I made a quick picture of some ideas I used when I built the bumper on my 3rd gen. I know the drawings suck but I did this during lunch on my laptop.
Lets start with the picture I drew on your truck. The black box is the main bumper channel which is not a problem or in question in this build .
Yellow= Random reservoir or whatever that is in the way of the frame rail on both sides.
The red is the bumper wing. The two small boxes on the bottom are supports that will extended to the frame rails.
Now to the picture on the bottom right. This is looking from the back of the vehicle. The red is the wing on the side. The yellow is still that thing blocking the frame rail. The black is the frame rail. I am hoping there is enough room underneath that they could just weld onto a bracket similar to the brackets used on bolt on sliders. If their is not enough room consider a bracket that looks like a backwards four, if that makes sense. It would have one part of the bracket that would hang below the frame rail that the support could be welded too.
Now for the picture on the bottom left. This is a view from above looking down on that corner. The black is frame rail, and again the yellow is that thing in the way. The red is the bumper and the bumper wing. The orange is an idea for another brace that could be welded to triangulate that corner and add a significant amount of strength for the wing.
I hope this made sense. Good luck and if you have any questions about my dumb scribbling feel free to ask.