Place a large kettle, pot or dutch oven over low heat.
Add salt pork. Brown salt pork, stirring, until golden, about 20 minutes.
Remove pork to a plate. Set aside briefly.
Pour grease into a heat-proof measuring cup. Pour 2 tablespoons back into kettle. Discard the excess or keep for another use.
Put cooked salt pork back in kettle.
Add onions, carrots and celery. Increase heat to medium low and cook, stirring occasionally, until they begin to soften and become fragrant, about 10 minutes.
Add garlic. Cook, stirring, for 2 minutes longer.
Drain clams, reserving their liquid. Set clams aside.
Place clam juice in a large bowl or pitcher. Add bottled clam juice and the liquid from the canned tomatoes.
Measure clam/tomato liquid. Add enough cold water, if needed, to make a total of 7 cups.
Add this liquid and the tomatoes to the vegetables. Stir to break up the tomatoes.
Stir in the tomato paste, ketchup, Kitchen Bouquet and a pinch of salt and pepper. Simmer, uncovered, for 45 minutes.
Add potatoes. Simmer, uncovered, until the potatoes are tender, about 25 minutes longer.
Add reserved chopped clams and the green pepper to the soup. Simmer 5 minutes longer. Stir the flour and cold water mixture into the simmering soup. Simmer 2 minutes longer.
Taste. Adjust salt and pepper, if needed.
Serve hot.