Weld 3/4" nuts to the outside of the U channel where the holes are torched.
Weld it with a bolt coming thru the other side so it protects the threads and so you know the nut won't get off center.
Grind away any spatter between the bolts.
Make (4) 3/4" x 3.5" bolts using 3.75" threaded rod and steel nuts. See Making Bolts.
Thread the bolts through the nuts and thread another nut on top as shown.
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