whisk eggs in medium bowl to blend. gradually whisk in milk. add 1/3 cup maple syrup, sugar, vanilla and salt; whisk to blend. divide custard between two 13x9x2-inch glass baking dishes. arrange bread in single layer in dishes. let soak 10 minutes. turn over; cover, and refrigerate overnight.
heat large griddle or large skillet over medium heat. working in batches, brush griddle with some butter. add soaked bread; cook until brown on bottom, about 4 minutes. turn french toast over; cook until bottoms are brown, about 4 minutes. transfer to plates. serve with additional syrup and warm berry preserves.