Shortly after releasing the 72" toolbox they claimed (on their facebook page, responding to a customer) that a top box for slated to be released in 2015. So far nothing on the website though....
You could get the 72" bottom and put a 56" top on it if you really wanted! A bigger toolbox is probably better
I say put the largest you can fit, I started with a little box, then a bigger one, and a bigger one, and then 2 big ones, and I still want another one. BUY the 72 and then buy a 56 to put next to it, I HATE top boxes, you can never have enough work space...
I have the 44" and I have had it for a year and a half and I am very happy with it. The only minor problem I had was the large drawer did not have a good solid detent to hold it in. I took the drawer out and added two magnets to the back of the drawer to help hold it against the back of the box. Works great. Nice box, I would recommend it.