In the large pot, cover the navy beans with water and allow to soak 8 hours or overnight. Drain and cover with fresh water and add the baking soda. Bring to a boil, then reduce heat and simmer for about 10 to 15 minutes. Drain the beans. (Quick soak method: Cover the beans with water, add the soda, and boil for 3 minutes. Then set aside tightly covered for 1 hour. Drain well.)
Put half of the diced salt pork or bacon and all of the diced onion in the bottom of the slow cooker pot, then add the drained beans. Sprinkle the rest of the salt pork/bacon around the top of the beans. Stir together the molasses, brown sugar, dry mustard, salt, and pepper, and pour it on top of the beans. Pour in 2 cups of boiling water and cover. Cook on low for 8 hours and remove the lid for the last 30 minutes or so to allow the liquid to condense a little before serving.