Check the beans for any stones or dirt and discard. Put the beans in a large bowl; cover with cold water by about 3 inches. Cover, and set aside in a cool spot or the refrigerator to soak for 6 hours or overnight.
Drain the beans. Put the beans in a large saucepan with 5 cups of water and bring to a boil over high heat. Reduce the heat so the beans simmer and cook for 30 minutes. Drain, but reserve the cooking water.
Preheat the oven to 250 degrees F.
Put the beans in a 2 to 2 1/2-quart ceramic bean pot or covered casserole with the salt pork and the onion. In a medium bowl combine the molasses, mustard, and salt with 2 cups of the cooking liquid and pour over the beans. (If the beans are not covered by the liquid add more.) Cover the pot, and bake for 3 hours. Check the beans periodically to make sure they are covered with liquid and if needed add more of the reserved cooking liquid. Gently stir in the tomatoes and vinegar, taking care not to break up the beans, continue to bake for 3 more hours. Remove from the oven and stir in the rum, if desired. Serve.
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