Rinse beans and pick them over. Put beans in a large plastic container with three times as much water as beans. Cover and refrigerate overnight.
Drain beans. Combine beans and chopped onion in a large saucepan. Add water to cover them and heat to boiling. Cover and let simmer about 30 minutes. Drain liquid into a small bowl to reserve for sauce and for cooking.
Measure 1/2 cup of the bean liquid into a bowl. Add the orange juice, molasses, maple syrup, ketchup, and prepared mustard. Then add the brown sugar, stirring to dissolve. Lastly, add the dry mustard and salt and combine.
In a bean pot layer half the salt pork and all of the beans, making sure to bury the peeled, whole onion in the bottom. Pour the sauce mixture over beans and add just enough reserved bean liquid to cover them. Top with remaining salt pork.
Bake, covered, at 300 degrees for 4 hours, checking on the hour. If beans seem too dry, add a little of the remaining bean liquid. Uncover and then bake for one hour more.