You will be able to tell how secure your bracketry is by having someone turn the steering wheel while you watch to see if the box moves from frame flexing. There is a lot of torsional force going on there.
This is a good idea for a test, but may not expose the loads of driving around corners. I.E. lean, weight transfer and such that the suspension my induce. My comments are supposition mixed with my background but I am not a steering box modification designer or pitman arm load engineer.
It is OK to box a short section of the frame if that strikes you. All this only if you find too much flex as Carl mentions.