In a bowl combine the beans with 4 cups cold water let them soak overnight and drain them, reserving any remaining liquid. Transfer the beans to a small heavy kettle, add fresh water to cover (about 31/2 cups) and simmer the beans, covered, for 1 hour. Drain the beans in a colander, reserving the cooking water and combining it with the reserved soaking liquid, and reserve the liquid.
To the kettle add a 1/4 pound piece of the pork and the beans and bury the remaining piece of pork, well scored, in the center of the beans.
In a small bowl stir together the molasses, sugar, 1/2 cup of the reserved bean water, the mustard, the paprika and the onion; pour the mixture over the beans, lifting the beans with a spoon to allow the seasoning to concentrate to the bottom of the kettle.
Bake the beans, covered, in the middle of a preheated 300 degree oven, adding some of the reserved bean water at hourly intervals to keep the mixture covered and lifting the beans to allow the liquid to penetrate to the bottom of the kettle, for 6 hours. (The surface of the liquid in the pot should just cover the beans.)
Remove the lid for the last hour of cooking.