The day before you plan to serve the chowder, place the clams in a large colander and rinse with warm water for 30 seconds. Working quickly while they're still frozen, shuck the clams, discarding their shells, and roughly chop the meat by hand.

Place the salt pork in an 8-qt. saucepan over medium-low heat. Cook the pork, stirring, until its fat renders, about 12 minutes. 

Add the bay leaves and half the onions and cook, stirring, until softened, 10 to 12 minutes. 

Pour the clam juice and tomatoes into the pan along with the chopped clams and bring to simmer. Cook, stirring occasionally, until the soup is reduced by one-third, about 1 hour. 

Stir in the potatoes and cook until tender, about 12 minutes. Remove the pan from the heat and let the soup cool to room temperature. Remove and discard the bay leaves and refrigerate the chowder a few hours, or, better overnight, or, alternatively, freeze up to one month.

When ready to serve, remove the soup base from the refrigerator or defrost. Place the bacon in an 8-qt. saucepan over medium heat. Cook the bacon, stirring, until its fat renders, about 6 minutes. 

Add the remaining half of the onions, along with the celery and garlic, and cook, stirring, until softened, 10 to 12 minutes. Pour the defrosted soup base into the pan along with the thyme and marjoram, bring to a simmer, and cook, stirring, until warmed through, about 30 minutes. 

Remove the chowder from the heat and season with pepper. Ladle into bowls and serve with oyster crackers.