Hello, you need to find out what the welder is rated at, not what the plug is rated, although you wouldn't think the plug would be rated under the primary. I'm guessing the rated primary current is just over 50 amps, let's say 56amps. For an arc welder, the ocpd can be no higher than 200 of the rated primary or rounded up to the next standard size (2x56=112 rnd=125). Your wire will need to be #6THHN, as the OCPD is also allowed to be 200% of conductor ampacity for arc welders.
If this is a resistance welder, see 630.32, you can go 300% of the rated primary current.
Hope this helps.
