If using bacon drippings, heat them in a cast-iron Dutch oven or large cast-iron skillet over medium heat. If using salt pork or bacon, cook it in a cast-iron Dutch oven or large cast-iron skillet over medium heat until crisp. Remove the pork or bacon and reserve for another use. Reserve the rendered fat in the pot. (You should have about 1 tablespoon drippings. A little more or a little less is fine.)
Add the onion to the Dutch oven or skillet and cook, stirring on occasion, until tender, about 5 minutes. Don’t let the onions take on any color. Pour in the clam juice and water, then add the pepper and bring to a boil. Add the chopped clams (if using canned clams, also add the liquid from the can to the chowder), reduce the heat to low, and gently simmer, uncovered, until the chowder clams are tender, about 1 hour.
Add the potatoes, increase the heat to medium, and simmer until the potatoes are just barely fork tender, about 10 minutes.
Add the clams in the shell, cover the pot, and cook for 10 minutes more, or until the potatoes are tender throughout and the shells have opened. Remove and discard any shells that remain closed.
If desired, add anywhere from a splash to 2 cups milk, half-and-half, or cream and let the chowder cook for a couple minutes more to warm through. Taste and, if necessary, season with salt. If you like, place a slice of bread each in 8 large, shallow serving bowls, then ladle in the chowder, making sure to get a couple shell clams in each bowl. Sprinkle with the parsley and serve with the crackers, if desired.