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?

Thanks for the picture – it makes things much clearer. Part of my confusion is vocabulary. The brochure for this boat https://www.sailrite.com/Balboa-27-Sail-Data says it has a 147 pound centerboard. Your saying the boat was without the iron keel threw me off. If the stub keel is still there with its ballast intact, then the centerboard slot should still be there too. If you want to do away with the centerboard, and not replace it, then the easiest thing to do would be to fill the centerboard slot with lead and cover that over with fiberglass. No need for bolts or screws that might cause leaks. No need for other metal that might lead to corrosion or galvanic issues. No need for added stuff underwater that would slow you down. Upwind performance without a centerboard may suffer, but as the saying goes: “Gentlemen don’t sail to weather.”.

New steel centerboard sounds like the best solution. Better stability when the board is down, from having some weight lower down. Better performance upwind from having a centerboard. Better results if you want to sell the boat from not having an “owner customized” (read “ruined”) version. Probably not too difficult to find & make, or have made. Do try to have forward edge rounded and trailing edge tapered, for better performance.