I had a lo profile WG crossover box in my '10 with a 5.5 bed. Did not leave room for much else. I believe it took up 20". I can't find any pics of my Honda Fourtrax 300 (small ATV) in there, but I think I did, back tires hanging off tailgate. Not sure length on dirtbikes, but I guess you could angle it in there. IMO, I don't think you need all that toolbox space. Maybe some sort of box that could be removable? With all the stuff you are in to, camping, kayaks, fishing etc, maybe removable would work best.