Place beans in a large pot and cover with 12 cups of cold water. Let soak 8 hours or overnight. Remove any beans that float. Drain, then cover generously with fresh water in the same pot. Bring to a boil then reduce heat and simmer slowly, covered, until tender, 45 minutes to 1 hour.
Preheat the oven to 250ºF. Grease a large casserole dish.
Drain the beans, reserving the cooking liquid. Place the beans in the prepared casserole dish along with the water, onion, molasses, ketchup, mustard, Worcestershire, and salt. Mix gently to combine. Top with the sliced salt pork.
Bake, covered, 3 hours. Uncover and cook another 1 to 1 ½ hours. If they begin to look dry add a little of the reserved cooking liquid.