The manual isn't the best for that, as they use the same diagram for both 120 and 240 and it really should be two different plugs. What they are showing is a 6-50. For 120 the white goes to the wide blade where for 240 the white goes to the narrow blade. Actually for 240 it doesn't matter which blade, but the wiring standard looking at the outlet with the ground pin up is white goes to the right. On 120V outlets this will be a wide blade, on 240V outlets this will be a narrow blade. The blades are swapped, not the wires between a 5-50 and a 6-50.
My local Home Depot has 14-50 plugs for about $14. Also They have 14-50 pigtails for stove installation that you could use as the start of an adapter, just add the appropriate outlet to the end. I went for the ultimate in short adapters and just installed the outlet right into the plug, that way I could carry it in the case.
Long arc, short arc, heliarc and in-the-dark!