Preheat the oven to 350°F. Spread out the bread cubes on a baking sheet and roast until crisp, about 10 minutes.
To make the bechamel, melt the butter in a skillet over medium heat. Whisk in the flour and cook on low heat for 10 minutes.
In a separate saucepan over low heat, warm the milk. Turn off the heat and whisk the milk into the roux. Return the heat to medium and bring to a simmer, whisking constantly. Continue to simmer (not boil) for 15 minutes, whisking occasionally. Season with salt.
Heat a large stock pot over medium heat. Add the bacon and cook, flipping once, until crisp. Transfer the bacon to a paper towel-lined plate, let cool, then roughly chop.
Pour out most of the bacon fat, and melt the butter in the pot. Add the onion and celery and cook, stirring, until softened but not colored, 5-7 minutes.
Add in the cooked bacon and wine and bring to a simmer. Stir in the clams, clam juice, cream, bechamel, and stock. Simmer for 10 minutes, then stir in the potatoes and cook for another 10 minutes, until fork-tender. Taste and adjust seasoning with salt and pepper.
To serve, divide the soup between bowls. Garnish with croutons and parsley.