Your sketch is very inaccurate, very misleading.
The 6x6x1/4 vertical appears very undersized to hold the two massive horizontal beams. In the sketch, you've "drawn" a scaled 10x10 vertical and two (undersized) 12x16X3/8 beams with a large gap in between the two horizontals.
result is your 6x6x1/4 vertical will be able to support (at most) only over 2-1/2 inches of each side. And that U-shape "end" of the square tube steel will tend to spread out (yield) as each beam pulls apart. The end cap will help some against this force on the vertical, but the two horizontals are only supported in the (thin) middle of the bottom surface cantilevered at both of their ends.
What direction is the beam (12x16) vertical or 16 x 12 horizontal main axis?