on my robbe u47, which was built almost exactly the same way as yours, (4 6 volt nimh packs and a 750ml engel tank, as well as geared 400 motors) i used the entire lead bar in the keel, but i had foam in the saddle tanks, as well as in the bow ahead of the WTC, and in the stern behind the WTC. the stern foam was cut so it would fit around the pushrods for the rudder and diving planes, the forward foam was cut so that it would fit around the forward diving plane pushrods and the "bolt" used to hold the two parts of the WTC together. lead wheel weights were placed strategically around the tech rack to keep the boat ballanced port to starboard. foam was held in place with double-sided servo tape so that it could be moved around/removed when needed. as jeff stated, most all of this was placed below the water line to give it a nice water line when surfaced, but more importantly, it was also arranged to keep the boat on an even keel with about 3/8" of the sail visible when the ballast tank was full. because of that, a lot of the stern foam also existed above the water line.