my 2 cents would be to replace the coolant every 30,000 miles and install a coolant filter. Otherwise, your rig is 13 years old so main thing would be preventative maintenance stuff and watching your engine vitals when you drive it.
If you have a monitor watch the spread on your ECT and EOT. More than 14 or 15F at 65mph on the flats and you're oil cooler is getting clogged. The mod that most people do is to either replace the EGR cooler with one from Bulletproofdiesel or delete the thing. If you have an early 2004 you might have the more robust EGR cooler design. Drive it like you stole it once in awhile to burn off the carbon in the turbo.
Otherwise, there are so many potential mods you can do to increase reliability it will make your head spin. Google heat management and EGR cooler on vans with a 6.0 powerstroke but here's a good overview: 6.0L Power Stroke Upgrades - Addressing the Engine's Fatal Flaws