I just moved a BP clone from Auburn, AL to my shop in Madison, GA. Used a gooseneck trailer so it was much easier to securely strap to the trailer. But it was no picnic. The guy I bought it from thought it would take about thirty minutes to an hour to load it. It took four hours (because of where it was located in his garage) and he had rented a boom-style forklift. Rarely does 'everything' go right on a move like this. I have a skid steer, and I had made a set of forks for it, but even with that, it was a little tenuous getting it off the trailer. All kinds of things can go wrong.
I would strongly suggest you figure out some other way to move it than in a box truck. For starters, how are you going to get it IN the box truck? I contemplated using one (as protection against potential bad weather during the move), but unless you have the right kind of equipment, you'll be risking serious damage and/or injury just getting it out of the truck. At the very least, you should use a box truck with a heavy duty lift gate, but even that is a long way from ideal. If you MUST use a box truck, you'll probably need to rent a forklift that has a boom to reach into the truck. Some have recommended using a boom-style wrecker or a crane to unload it. While good advice for a flat bed truck or trailer, I can't see how that is going to be of any use to you on a box truck.
Several posts have offered some valuable advice on how to move the mill. Some of the best being to drop the knee to its lowest level (I would add moving the table as far toward the back of the mill as it will go), and do a 180 on the head. That lowers your center of gravity, but it's still a bear transporting something that heavy. Depending on your shop layout, you may want to consider renting or borrowing a pallet jack after it's in the building. As has been suggested above, you can roll it around fairly easily with just two pieces of round stock and some sort of lever, but a pallet jack is much more maneuverable (a little challenging to get out from under the mill when it's at its final resting place, but quite doable).
Best advice anyone can give you: BE EXTRA CAREFUL!!! That machine weighs around a ton and is very top heavy. Think it through carefully, and don't rush it.
Regards,
Terry