This is the same AMT kit I used to build the T600 full aero cab. This one has the small sleeper and partial aero package. I cut the sleeper off and used a small walk-in sleeper from the parts box.
The T600 was a radical shift in truck design from W900. The major difference between the two was the massive aerodynamic face lift the T600 received, which resulted in up to 22% better fuel economy due to decreased drag.
The design work for the T600 began in the mid 1970s in wind tunnels. The T600 was replaced by the Kenworth T660 in 2008, which featured smoother styling. To me the easiest way to tell the differences is the headlights.