An across-the-box toolbox that fits a Ranger (or mid-size truck) will fit. I had one on mine. I liked it, and only took it off because I got a topper. The problem is fastening it to the truck. I drilled holes in each end of the toolbox and bolted it in using the threaded holes already in the truck box for the factory cover. Finding where to drill was a crude trial and error, so it doesn't look neat, but I never had water get into the toolbox. There's space between the ends of the toolbox and the side of the truck. You could bolt a piece of wood to the truck and run screws through the toolbox into the wood. Wish I would have thought of that 2 1/2 years ago instead of now!
You'll still have some space below the toolbox, depending on the depth of the toolbox you get. It cuts down on what you can haul, but less than you might think. You're adding storage high in the truck box in space that mostly doesn't get used. One drawback is you can't see the back of the truck, even with a low profile toolbox, due to the short truck box.