1 pound pasta shells
1 10.75-ounce can condensed Cheddar cheese soup
2 cups whole milk
2 12-ounce cans evaporated milk
2 tablespoons butter, melted
2 teaspoons Worcestershire sauce
1 teaspoon mustard powder
1/2 teaspoon freshly ground black pepper
2 cups shredded sharp Cheddar cheese
1 cup shredded Monterey jack cheese
2 garlic cloves, minced
12 slices cooked bacon, chopped and divided
Sliced scallions, for garnish (optional)