I mounted a toolbox on mine and drilled holes through the side of the toolbox and screwed 2 6 x 60mm bolts into the existing threaded holes in the truck box. (6 x 65mm bolts would be ideal if you can find them. The 60mm bolts can be tough to get started because they aren't much longer than necessary.) That's 2 bolts on each side. The threaded holes probably have covers on them you'll have to pry off. If the toolbox is deep enough, you'll have to remove the tie down anchors from the bed. If the toolbox isn't that deep, you might be able to hook into the tie down anchors with J bolts through the bottom of the toolbox. Might have to flip the anchors upside down.
Others have said there aren't any toolboxes that fit a Sport Trac. Any box that fits a Ranger fits a Sport Trac. They're referred to as "mid-sized" toolboxes. Our truck beds must actually be a little wider at the top than Rangers since there is about 1 1/2" space on each side between the body of the toolbox and the side of the truck bed. Our beds taper from top to bottom, so there's not much gap at the bottom of the toolbox.
For those in central Iowa, the Dee Zee Outlet store in Des Moines sells new scratch and dent toolboxes for $50. They have various styles in both steel and aluminum.