How should I use the ground wire for this welder?
I have one of these, and I dug out the manual (printed October 1987). It has a sentence that reads: "The center contact in the receptacle is for the grounding connection. A green wire in the input cable connects this contact to the frame of the welder. This ensures proper grounding of the welder frame when the welder plug is connected to the receptacle". So its just a ground, and the existing neutral wire could be used.