there are crossings so you may need four sensors at all four corners of the bottom to track the path.
there are turnings so the distance of the cube may vary slightly, try to get maximum and minimum distances during this by the data provided, then you should make sure the sensed LED doesnt turn off before it passes the block.
consider at curves there wont be any perpendicular surface to reflect perfectly when passing white blocks. thus some fine tuning of sensitivity is required.
at gaps the field is too far, and can be recognised as black block, so you may need two types of sensors here, one is to detect there is an "object" and another one is to detect "no white surface", when both occurs at the same time only it should indicate sensed.
so both side of the robot will have to have 2 sensors, for both object sense and for intensity/colour sence.