Hammered, Mike made some good points and I'm guessing you will be fine with 10 AWG since your distance is relatively short - though it is hard to tell without knowing the welders current draw.
dirtyfarmer, your cord looks good to me especially since the ground connection internal to the welder is chassis mounted, so the combination of the 6 AWG and 12 AWG should make a more than adequate HF ground.
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