Preheat the oven to 400 degrees. On a baking sheet, arrange the bread slices in a single layer and drizzle with the oil. Bake until the bread is toasted, 7 to 8 minutes. Rub the bread with the whole clove of garlic; discard garlic.
In a large saucepan, cook the bacon over medium until crispy, 5 to 7 minutes. Using a slotted spoon, transfer to a paper towel-lined plate.
Pour off all but 1 tbsp. of the bacon drippings from the saucepan. Add the onion and chopped garlic; cook over medium, stirring often, until the onion and garlic start to soften, 1 to 2 minutes. Add the sweet potato; cook, stirring often, until coated, about 1 minute. In a bowl, whisk the milk, clam juice and flour; pour into the saucepan. Increase the heat to medium-high and cook, stirring constantly, until the mixture just begins to boil. Immediately lower the heat to medium and simmer, stirring occasionally, until slightly thickened, about 5 minutes. Add the cod and three-quarters of the bacon; simmer until the potatoes are tender and the fish is just opaque in the center, about 5 minutes more. Season.
Top the chowder with the remaining bacon. Serve with the garlic toasts.