You can get an idea of how much the body moves in relation to the chassis looking at this shot of mine taken by Alex on the Cox's river run.



The plastic fantastic (not) front bumper is chassis mounted and the wheel flares are body mounted. The gap between is almost nothing on the passenger side in this shot and still maybe 1" on the drivers side. The gaps are normally equal.

This movement is over exaggerate by looking at the bumper/flares as the flares are a lot further away from the body mounts and will therefore move a greater distance as they effectively move in an arc. The amount of movement seen at the sills will be less as they are much closer to the body mounts. If you do go for the chassis mounted ones then I'd say they need to be kept maybe half an inch away from the sills to be safe.