Sort and wash beans; place in a large bowl. Cover with water 2 inches above beans; cover and refrigerate overnight. Drain.
Place beans and salt pork in a large Dutch oven; cover with water 2 inches above beans. Bring to a boil. Cover; reduce heat, and simmer 30 minutes. Drain, reserving 2 cups liquid. Cut pork into 3 slices, and set aside.
Combine molasses, catsup, sugar, Worcestershire sauce, mustard, salt, pepper, and onion; stir well. Stir in beans, salt pork, and reserved liquid. Pour mixture into a 3-quart casserole. Cover and bake at 250° for 6 hours. Uncover and bake 1 additional hour.