First, your Diamond-REO looks really nice. The licenses plates should be on the front, not the back. Anyway, a dump truck? Well, there are a couple of dump truck kits out there: the AMT Autocar DC9964, International PayStar5000, & Ford LNT8000 come to mind. You might have to scour eBay to find them, though. To make one you could use the Diamond-REO that you just built (well, another kit, not that ONE), the AMT GMC General, AMT Chevrolet Bison, AMT Mack DM600, DM800, & R600. I've also seen Peterbilt 359 & Kenworth W900 dumps (Revell of Germany makes a W900 dump). Depending on what time frame, and what appeals to you just about any kit can be made in to a dump. Although the Kenworth T600 and International ProStar or LoneStar probably won't make very good dump trucks.
Here is a good reference site: http://www.hankstruckpictures.com/
Have fun!