In my honest opinion, I think that 6mm wall thickness on those lengths is way OTT. You'd have to use something like M16 or M20 bolts to match the shear strength of those sections. It is generally poor design to pass a bolt through a box section as it will crush the walls unless you use reinforcing tubes as suggested earlier. The next option is to drill a clearance hole in one face and use short caphead bolts and nuts so that you're tightening face to face rather than across the section. If you insist on having threads in the box section, you could also weld a nut or threaded block inside the box section. Or you could use rivet nuts: How to install a rivet nut (rivnut) and the tools you need - YouTube