1. Adjust oven rack to middle position and heat oven to 350 degrees. Bring 3 quarts water, beans, and baking soda to boil in Dutch oven over high heat. Reduce heat to medium-high and simmer briskly for 20 minutes. Drain beans in colander. Rinse beans and pot.
2. Cook salt pork in now-empty pot over medium heat, stirring occasionally, until browned, about 10 minutes. Add onion and cook until softened, about 5 minutes. Stir in water, beans, sugar, 1/4 cup molasses, Worcestershire, 1 tablespoon mustard, vinegar, and 1/4 teaspoon pepper and bring to boil. Cover, transfer to oven, and cook until beans are nearly tender, about 1 1/2 hours.
3. Remove lid and continue to bake until beans are completely tender, about 30 minutes. Stir in remaining 1 tablespoon molasses and remaining 1 teaspoon mustard. Season with salt and pepper to taste. Serve. (Beans can be refrigerated for up to 4 days.).