After looking at the cliff uvs, the only way to get rid of the seams left by the box map is to unwrap the cliffs. To my knowledge, using a box uvw will always have some kind of uneven or ugly seam. I would unwrap it but I don't have much time. Sorry :\

Besides that though, the level looks awesome. I played it for the first time tonight and was blown away at the amount of visual detail. I wasn't to detracted by the cliffs, but some spots were noticeable.