Adjust oven rack to lower-middle position; heat oven to 300 degrees. Add salt pork and bacon to 8-quart Dutch oven; cook over medium heat, stirring occasionally, until lightly browned and most fat is rendered, about 7 minutes. Add onion and continue to cook, stirring occasionally, until onion is softened, about 8 minutes. Add 1/2 cup molasses, mustard, beans, 1 1/4 teaspoons salt, and 9 cups water; increase heat to medium-high and bring to boil. Cover pot and set in oven. Bake until beans are tender, about 4 hours, stirring once after 2 hours. Remove lid and continue to bake until liquid has thickened to syrupy consistency, 1 to 1 1/2 hours longer. Remove beans from oven; stir in remaining tablespoon of molasses, vinegar, and additional salt and pepper to taste. Serve.