Dogfish, Use the black, the red, and the bare wire with a 3 prong plug.
Cap the white wire off and push it in the back of the box and leave it.
I have been a liscenced electrician for almost 20 years. You dont even have to hook the white wire up in the panel if you dont want to.
The white is a "Neautral" which you would only use if your welder was a 220volt welder with a 120 volt roll feeder or something like that. All you need is the 2 hots (black and red) and the bare which is a ground.
If you need to ask any questions I sent you my number in a PM.
Tom