Your comments have me thinking about a couple of things. Since these units are quite heavy (55-80 lbs) they need to be supported by the deckhouse framing, not the steel plate skin. So additional frames can be extended all the way to the window openings.
And if they are full-penetration welded to the support frames then they can be partial-penetration welded to the plate skin (butt welded). That should help to control distortion as well.