The specs on the rims are key to knowing if you need a BMC or not. Just glancing at it... You should be able to align your way out of the rub. More positive caster should do the trick, but hard to say if the truck will allow that (they are all a little different). I would take it back and see if they can squeeze another degree or two of positive out of it. The more the better 4+...(ignore the recommended factory specs for caster which I think is 2-4) Small changes make a big difference in clearing the body mount.
If the rims are too wide or have too little backspacing you will need the BMC. Cutting the body mount without welding is fine... Boxing it is the right way to go, but it will hold up just fine with only a cut... (although you usually want the person welding it to be the same person cutting it, so you don't leave them a mess) I just would not do a BMC unless it is needed it for sure. There are some possible safety concerns in a very specific type of crash. I did mine and don't worry about it, but my tires absolutely required it. 285s usually don't.
