I'm planning out the materials for a rear bumper. The main purpose of the bumper is not so much protection, but facilitating recovery- a spot to jack with a HiLift, mount shackles etc. I don't rock crawl, but I do drive on deactivated forest service roads with washouts. My stock rear bumper has bent up and hit the body a couple times. I'd like someone with some fabrication experience to check over my materials choice (at the bottom) and make sure I'm not off track. I'm modeling it on this bumper by Irbis.







It is mounted to the frame similar to this:




I plan on using:
2x 4x .180 rectangular tubing for the piece parallel to the tailgate
2x 2x .120 tubing for the side panel protector (not the round tubing)
3/8 plate for the frame mount

There will be a backing plate which slides into the open end of the frame. I will likely use 3/16th or 1/8 for this. The nuts will be welded to this backing plate to eliminate the need for a second wrench.

Questions:
What size bolts should I use? 1/2" grade 8 work? See above pic where the mounting plate is bolted to the frame.
Could I get away with .120 (instead of .180) for the rear cross-piece? Remember, I don't rock crawl, and the shackles attach to the mounting plate which extends through the bumper.
If I added lots of gusseting, could I get away with 1/4" for the mounting plate?