Not quite, Cap! We do a lot of steel fabrication down here in the 'oil patch'. And 'steelwise' we are a lot more generous than you are. Since your are very good at welding; I would add 1/4" thick 'brackets' shown in red (yours are on the 'skinny' side). This would beef up the X axis section modulus and should take care of vertical forces. I am still worried about the Y axis section modulus and horizontal bending moments. Go easy backing up or stepping on the brakes.
It seems like a steel plate a couple of inches wider than the tube welded onto the top of the tube would eliminate horizontal bending moment. To reduce weight it could even be designed in a pair of triangles. Just something to make the center of that tube stiff.