I need to build a storage box that is 6 inch high by 66 inch long by 24 inch deep that can withstand a weight of 72 pounds. What kind of wood should I use? Thank you for your help.
Depending on your exact requirements and a few details of the final design you could use many different woods for this, possibly including pine if the centre of the 66" span is supported in some way or otherwise stiffened.
But I think the perfect solution for this would be to use a readymade board material, with plywood topping the list. MDF can also be a very good option, but may also require stiffening so as not to sag across the width when the box is fully loaded.
Cheap plywood these days can be very poor, so for strength and a good finish it's usually worth paying a bit more for quality stuff. 'Birch plywood' is often the thing to look for and this can be sold as 'cabinet plywood' or 'cabinet-grade plywood'. Sorry to say that unfortunately none of these are absolute guarantees of quality any more so try to buy in person and select the boards individually, don't just take the first one or two off the top of a stack.
This Old House had a design for a platform bed with storage underneath that was basically a set of three five-sided MDF boxes, two opening to the sides and one at the foot of the bed, with two internal dividers to make the space more useful and help carry weight. Since then I've seen a similar design based on very slightly modified Ikea bookcases, which I think are melamine-coated chipboard or something similar rather than MDF; there, the fixed shelves acted as the dividers/supports. Not exactly high-precision engineering, and more than strong enough for any semi-reasonable distribution of weight.