Tis difficult to aviod some distortion on this type of weld
I kept distortion to the very minimum by just tacking the square tube to the plate, I didn't fully weld untill the whole upper mounting assembly was all in place.
The only things I welded fully before assembly were the weld nuts & shock brackets.
The shock bracket does hamper things when fully welding in situ but with the right Mig welder set up, a meaningfull weld is possible
Looking at that top picture, have you fully welded both sides of that square tube ?