I'm not 100% sure what you mean - I'm assuming you are talking about the overhang of the box vs the sides of the truck. Ours overhangs very little, so as you mentioned, we'll need to have the brackets extend out quite a bit to support the box. We plan to have a friend weld up some plates to the trailer jacks that we ordered:
https://www.overtons.com/overtons-square-tube-fixed-mount-trailer-jack-315525.html
We ordered the jacks a few days ago and will be doing the project fairly soon, as now that we have the box off of the truck (borrowed the winch jacks from the guy we purchased the box from) we will need these ready asap to get it back on the truck.
I will update when we do that. We have even talked about doing a build post, so I'll link to that if we do.