Combine tripe and 1/2 gallon milk in a non-reactive bowl. Let soak in the refrigerator, 8 hours to overnight. Drain milk and rinse tripe. Return to the bowl and cover with remaining 1/2 gallon milk. Add onion, garlic, and bay leaves. Let soak in the refrigerator, 8 hours to overnight. Mix sour cream, scallions, Worcestershire sauce, English mustard, garlic powder, and cayenne pepper together in a bowl to make dipping sauce. Chill until flavors combine, at least 1 hour. Drain tripe, discarding milk, onion, garlic, and bay leaves. Rinse tripe and pat dry with paper towels. Cut into tortilla chip-sized triangles. Heat oil in a deep-fryer or large saucepan to 350 degrees F (175 degrees C). Fry tripe in batches until puffed and golden brown, 1 to 2 minutes. Drain on paper towels. Season with salt and pepper. Serve dipping sauce alongside fried tripe.