Rinse and sort beans. Cover with 6-8 cups of water in large pot and soak overnight or at least 6-8 hours. Drain and rinse beans before using.
In a large cast iron pot, cook salt pork in oil over medium heat until rendered. Add onion and cook until tender and translucent. Add molasses, tomato juice, mustard, sugar, and garlic. Mix well. Add beans, then add enough water to cover beans with 1 inch of water, about 2 cups.
Bring beans to a boil and cook for about 5 minutes. Remove from heat, cover, and bake for 2-1/2 to 3 hours at 325 degrees or until beans are soft and the sauce is reduced to a thick liquid. Check the beans every hour. If beans become dry, add additional water.