Check the ratings of the "SO" cable. They carry a lower rating than standard cables. 6/3 SO/SOO cable has a rating of 55amps as opposed to 60amps in regular cable.
I suggest 4ga/3c (2 hot conductors and a ground) to cover for the voltage drop as mentioned earlier.
You mentioned it was 220v, so I'm assuming it's a single phase welder.
Will
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