Interesting question Troy. I am reasonably certain that your engine is a pre-DDEC model. Unfortunately, that leaves several posibilities for type of governors which control the idle speeds. Any idea which type your engine has?
I would suggest that you ask that question on
http://autos.groups.yahoo.com/group/DetroitDiesel/
These guys eat, sleep and breathe the two-stroke Detroits. If you are not a member, sign up and they will need your engine ID number which is located on the block on the drivers side.
Your idle speed is good (500-600), high idle should be in the 900-1000 range.