600 miles? Realistically probably not near what the person who owns it thinks its worth. If it was a L9000 or LTL model that would change it some.
Need a little more info, What Cummins engine family, what trans does it have, what drive axles and rear suspension plus what lbs the rear suspension
and front axle are rated for. What style of wheels it has, cast Minnesota Mags or steel, hub pilot or stud pilot. What brand of dump is on it? Some are
better than others.
But remember on vehicles that sat and not run, the hoses need checking, tires may have age cracks, Water pump will probably start leaking. Things like
wheel seals and any other seals can start leaking due to sitting and not rotating. Plus the fuel in that puppy might be pretty sour. IMHO
to add to Truck Shop's list, you will probably need to change most of the air valves, relays etc, I wouldn't take it far without new steering tires (shame to wreck the hood with an age blow out) look closely at the fuse box for signs of corrosion, (if the batteries were left connected and age dies the corrosion can spread) If the truck has been started periodically quite a few of these problems will be less likely, keep in mind that you might have to replace most of the rubber hoses, and maybe the fuel lines.