Well they discontinued the 7.3 in 2003 but it does usually take more than a year to build a bus.. .So in the spirit of 0 being 1 for very high values of 0, it is possible that the chassis was built in late 2003 and it took until late 2004 to build the body.. possibly making it a 2005 model year bus on a 2003 model year chassis.
But yeah, more likely it's a 6.0.