John, I have seen and been aboard the one and only example of what George Kittredge calls his best submarine--the K-600. The hull and ribs are 3/8" and the viewports are heavier, but I think everything else translated over from the K-350. I have not seen a set of K-600 prints, but I would build it given the option, rather than the 350, just for the additional flexibility. Depends on your purposes. Vance