Your van was designed with a wider front than rear. For offroading, that can be a problem in that the front track is slightly wider and hence the rears might hit something the fronts don't. But that's probably insignificant on a big ole SMB. Especially considering that adding spacers adds a signifcant failure point on the van. When I used spacers on the track I had to constantly inspect them. I've seen wheel on spacers come flying off a SMB offroad, and go sailing off down an embankment. Not pretty. The thought about bearings is that since weight is moved out on a lever, it puts more stress on the bearing. Seems intuitive, but I doubt you'll find data.
I suspect the difference isn't that large. If you feel the need to make the rear bigger, consider as a first option getting front and rear wheels with different dishes so as to bring them more in alignment; you might be able to get them equal without impinging on brakes and without the need for spacers. Indeed, you might be able to bring in the front enough to match the rear.
