Better to be big on your wire size. The 50amp outlet allows for a larger gauge wire. You don't have to have 50amps going thru it. It's the MAX for that outlet. 30amps to 50amps can be carried thru that outlet. Much easier installing #10 in a 50 outlet than a 30. Also, less resistance on the line with larger wire.
When your first commented I was concerned you only had a 20amp 110v welder. Those smaller welders cannot draw more than 30 amps.
One last thing, make sure your ground wire is connect all the way thru your circuit and connect to each box (casing connection) as well. Do you have a ground rod or mat at both your main panel and the sub panel? I just checked mine last week and there is no ground rod at my sub panel out here at the shop. So if I get a short it's going into my house panel and then into that ground rod (if it exists). :((
