Well the 03 was the first year of the 6.0L and it was a disaster for Ford
they had some major problems with the wiring harnesses, along with head gasket's. But on the other hand if properly deleted and upgraded they have great power, and with the turbo having the vanes that change you get some killer turbo whistle
If it was me I would as soon as I could EGT delete, and do the necessary upgrades too the oil cooler, and get ride of the Ford coolant and replace it with a ELC coolant, also a coolant filter as a add on.
There is a TON of information at places like Powerstroke.org, The Diesel Stop, and Powerstroke Nation just too name a few, knowledge is key as well as maintenance too keep your truck running reliable.