I would not mount anything to the bumper. I had a bumper come off (broke welds) on I 95 many years ago. I drug one side of it until a passing car got my attention. I broke the other side off and put it in the truck. The bumper is thin metal and the frame is thicker metal. That leads to welds that don't burn through the thinner metal but they don't penetrate the thicker metal all that well. It's the nature of welding thin metal to thick metal. It would be stronger if it were bolted on IMHO.
I think brackets bolted on would be better than nothing. But welding something made from same thickness to the frame with good penetration welds would be best.
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2016 36FBTS Pinnacle
2016 F350, 6.7, 4x4, DRW, long bed
B & W Companion 5th wheel Hitch