From the looks of the picture, there are two upper bolt that mount the box to the frame (and either one or two lower ones, hidden from view). Beside the upper bolt (the one closer to the radiator / further from the bumper) , it looks like the frame (the upper surface) has been clearanced or cut. This may have been done because the placement of the plate puts that bolt in the same place as the top surface of the frame rail (two objects can't occupy the same space at the same time).
Some might argue this set-up would be okay, but the frame in this area is going to see lots of loading. AT THE VERY LEAST, I'd spend an hour welding a plate on the top-side of the frame rail to cover the gap, get rid of a stress concentration, and distribute the load better. If you wanted to finish the job nicely, you might consider plating the outer surface of the frame rail (looks like the inner surface has been done), sleeve the front cross-member where the box/steering shaft pierces it, and maybe even box in the right hand frame rail (in front of the cross-member).
You've got the right idea fixing the goofy previous owner stuff right away...
Cheers,
Steve