What do you mean by, "box in the back"? I'm not familiar with that. Do you have an example of what you mean?

I do plan to drive it on the street. I'm not familiar with a gusset, but I'm assuming it means to take a piece of metal that will fit between the back of the ladder bar brackets and weld it in between them so that it will close that area and also prevent it from flexing as you mentioned. Is that right? I've seen people weld a thin plate on the bottom of their lower control arms that I'm assuming serves the same purpose. Would there be any benefit to gusseting both sides while I'm at it?