Why put lead into SS square tubes? Pictures might help. The boat is supposed to have more than a ton (2400 pounds) of ballast in the stub keel that you seem to say is missing. Attaching that much weight to the bottom of a boat that may have been damaged when the original stub keel was removed could present problems. Whatever you end up doing would need to be engineered carefully. You would not want the new encapsulated keel torquing off the boat and preceding the hull’s trip to the bottom by just a minute or two. Not having the centerboard may make going upwind a problem as well. What happened to the stub keel?