Ok. This is what I would suggest (as someone who used to go rock climbing with my jeep):
1. Cut out (same gauge plate) a square and get someone to tack weld it behind the 2 plates (engine side).
2. Also, cut a strip and tack weld it above the plates (it can be a thin strip on top towards the engine side of the top that goes across like a hat). Make sure you can still insert the upper control arm into the bracket after. See picture below.
3.Then remove the boxed bracket, have it fully welded. It now will have strength in left and right, and front and back. It shouldn't take much money, or time.
4. Once it is fully welded, I would weld it to the frame. If you weld it, you really only need run two beads across the frame on either side of the boxed plate where it sits on the frame. Keep using the bolts, but the welds will prevent movement, which will eventually happen. If you only weld there, it would be very simple to remove later (If needed) with an angle grinder but cutting at the welds.
This is what I would do, but it is up to you. But either way, looks good!
See Picture: (Red plates, and welds on each side of the bracket)