When your torque the nut, does the cotter pin hole line up with the castellations? If everything lines up, then you don't have room for any spacer, regardless what the directions say.
I didn't particularly care for the lack of detail in the CE crossmember installation instructions. I would have liked to have had specific instructions as far as where to put bolts, washers, nuts, torque specs, that kind of thing. As it was, it didn't take too awful long to get the crossmember installed, but it could have gone more quickly.