They do come off!

Long story short, the sides came off the grain box today. Dad-gum-it, they were heavy! I mean, really heavy! The front is all that remains, and while I won't say it's welded to the bed (since I thought the sides were welded to the front), it is a solid connection between the two.

I do have to say - I love the look without the grain box on there, and just the flatbed left behind. I still want to add the stake side, but man, what an improvement!