You could fill a tire with that tank. It would be hard to run any tools with that tank unless you have a way to replenish the air in the tank. I'm not sure what kind of compressor you have with the air horns, but it would need to be capable of maintaining the pressure in your tank. You could run them for brief periods until the pressure dropped too much.
Were you planning on mounting this on the truck, or just keeping it in the bed? I'm thinking of rigging up some on-board air and looking for ideas on where to mount the tank.