Just because it's a grain truck doesn't mean you can't haul gravel or dirt in it. Farmers do it all the time. You just can't fill it FULL. You can haul 3 or 4 cubic yards, maybe even a little more. As others have mentioned, you can always shorten the bed by a foot or two if dumping is a problem. (This could also vary, depending what you are hauling. Some types of rock or gravel would slide out easier than dirt might, for example.)
