I've got a 99 F350 with a 7.3 and it has 433,000 on it.  I can confirm that it's the original engine, runs good.   Fuel gauges on those pickups are notoriuusly inaccurate as I discovered a few months after buying it.   I had to pressurize the fuel tank a few times to get it started after running out.   Run it hard and get the air out and you'll have a good vehicle.   Those engines run forever.