I would recommend 0.120 wall thickness or higher, so far I haven't had any bending or cracking (fingers crossed). I pretty much followed and old article in 4wheeler for building a prerunner tube bumper. Off the top of my head 1.75" or 2" round tube for main structures and 1.5" for supports all 0.120 wall or more. I went with 2x4 tubing because of the swingout posts I bought from Ruffstuff. If I were to do it all over again I would do a double shear mount like what Relentless Fab sells. The main problem with the double bearing pivot is setting the final height of the outer bearing race (the bigger tubing attached to the swingout). To fully assemble the pivot you must press in (Or lovingly hammer in) the upper and lower bearing races, but the main issue is when you go to weld it all together. Unless you go slow I would remove the races before burning in the swingouts, I didn't have a way of doing that or the time. I eyeballed the height and adjusted it by cranking down on the castle nut lowering my bearing height. *Side note: The tolerances for the bearing races were SUPER tight, i had to waller her out a bit to set the races in but she holds and works great so far.
You can get as fancy as you desire with a bumper build, I wanted to since I have the bender but then weight became a concern and it adds up fast, so keep that in mind.