I would recommend going to at least 50 amps as well.
BUT...with welders you can get by with less. That is because homeowner welders have a duty cycle on them, so they cut-out before wiring overheats by their very nature, so you can get by with less amperage or wire size. The welder outlet should have a warning label on it though saying "Welder Use Only".
I would go to 100 amps though just because if you got to cut into the walls, you minds well go big or not do it at all. That way if you want a back up, or standby generator down the road, you will have the ability to tap into that.
But that is the problem with these projects, the idea of doing it once can mean doing a lot (and a lot of expense).