All you need is your 2 hots and a ground. If you look in your panel, you will see that your neutral and ground bus bars are connected and bonded together. You only need your red and black to the breaker poles and a neutral or ground wire. Best thing to do is change the receptacle to match the chord on the welder and change your wiring to match as it wouldn't be wise to have an unused wire in your panel and probably against code.