At one time, I wondered the same thing myself - they don't cross-fit, as already indicated. I don't recall about the overall length of the action, but I do know that the magazine well is significantly wider, and they modified the shape of the WSM stock a bit in the forearm as it is wider and thicker there as well. If memory serves, the WSM stocks also have cross-bolts for the added recoil where the .308/III action size does not. I would guess removing any material from the slender III stock to make it fit the WSM size action would almost guarantee a short usable life.
