2 or 3 wire for welding hook up

Hi I'm going to install a 220 welder which comes with a 250 VAC 3-Prong Plug (6-50p). I need to install the corresponding 3 prong outlet. I know it requires 2 110 volt hot wires and a ground for the third prong (usually the green wire in a stove hookup). My question is whether I need to pay extra for the 3 wire or can I just use the un-insulated copper ground wire for the third prong (which does indeed require a ground connection) and just buy #6 - 2. Is the insulation important if the wire is hooked up as a ground? Thanks Elliot