Playboy, there's an option you might want to consider, although it is considerably more work.
If you weld nuts or bolt heads to the inside of the frame, box over them, and they strip or break, you've got a problem.
Consider welding a sleeve in between the frame and the box plate. You end up using longer bolts, but you can make them about any size you like, and, if something gets broken or stripped, no big deal.
