forget mesurements, the ones measuring hieght are useless, you only need to measure the internal dimension for the board length, then after you get the floor board on, stick a tub with the lid on it in there, then set a ruler on the left and right side of the tub, lay the board on top, mark, level, and drill. if you need to hold the thing together while doing all this, cut the sides about a foot taller then you nedd them and drill in a 2x4 on the front and one on the back (if it were standing up), and after you get your final floor board on, mark the top board, and before you drill it, take the whole thing and cut off what you dont need from the top.

but if you want to do it your way, then here goes, if the bottom floor sits between the two sides, then the sides will need to be 72.55, if it sits below the two sides, the sides will need to be 71.8, and since thats gonna be a b**** to measure, set it between them and give yourself a slightly less painfull headache good luck! i hope i helped