I almost did the same but realised when it was getting too tight as the bushes on the bar were deforming so loosened them off. As someone said previously, they are nyloc nuts so shouldn't move anywhere
Problem i saw is that if the brackets arent sitting against the beam, your allowing the mount to 'rock' on the 'U' bolt?
Mine is tight enough for the brackets to sit flat against the beam, some deforming of the bushes, but more stable and double nutted!
Jimski
can do it by hand having a jack underneath to hold them in place would make things a bit easier though.
Someone at Whiteline probably just put the wrong washer in the bag by mistake, i doubt it will happen to anyone else!