Combine brown sugar, butter, and corn syrup in a small saucepan. Heat over medium heat until blended, stirring frequently. Pour mixture into a greased 9x13 inch pan.
Arrange bread slices in a single layer over brown sugar mixture.
Drain peaches; spread in an even layer over bread slices.
In a medium mixing bowl, combine milk, half & half, eggs, cinnamon, and vanilla extract. Whisk until blended. Pour mixture over sliced peaches.
Cover and refrigerate several hours or overnight.
Before baking, prepare streusel topping by combining butter, brown sugar, and flour. Sprinkle evenly over french toast.
Bake, uncovered, at 350° for one hour.