Like Jon said, we need to know more about your 580. If it's a straight 580 with no letter designation, it has a RoosaMaster/Stanadyne injection pump on a 188 diesel engine. These pumps continued until Case started installing Consolidated Diesel engines in the 580's starting with the 580E I think.
If you have a leak around the throttle shaft, chances are that you need more than just a seal or oring. You probably need a new bushing in the housing as well. If you're very adept with mechanical things, there are manuals online that can help you as well as parts that are available for the Stanadyne pumps.
However, if you're NOT pretty experienced with tedious and meticulous mechanical repairs, remove the pump, have it repaired by a diesel shop and reinstall it. Serious engine damage can occur if these pumps are not repaired correctly.
Mark