i have that same bottom cabinet, bought it in 93. it's a matco brand but the same company makes all of sears and the tool truck boxes. it is a nice box, big enough to hold a lot of tools but not so big you can't move it.
i use to do plant maintenance and had to roll to fix machines. i never bought a top box for it because i liked having the top as a mobile work surface.
lot's of bigger tool boxes on craigslist.net. look there before you buy a bigger tool truck box. lots of laid off mechanics looking to raise some cash.