Ben, IMO stitching images from UWA lenses is just asking for trouble. You are better off using a more moderate lens, like a 35 or 50, and taking more images to encompass the sweep of the vista (and multiple rows, if necesary). Also, you might reconsider the composition: do you really need to grab all that real estate, or could you find an effective compositon from a cropped view?
Why does your BH have to flop over when shooting verticals? Do you not have an L-bracket? If not, get one.