Soak beans overnight. Bring to a boil in same water and simmer until tender, 1-½ to 2 hours. Drain, reserving liquid. Preheat oven to 275 degrees F. Place beans and salt pork in casserole or bean pot. Combine cooking liquid, brown sugar, mustard, onion, molasses, and salt. Pour over beans, cover, and bake 3 to 4 hours.