If that's the style I think it is, they don't want you using the two breaker positions directly opposite the Main Breaker, and they'll have a "Do Not Remove This Knockout" sticker on those two breaker KOs. They are already pumping 100 Amps through those stabs which is about all they can handle, they don't want a load breaker there to keep from melting the stabs. Partly exposed, they'll get some cooling from air circulation inside the can.
I would pull the dryer breaker (save it for when you move) and use a Bryant BRD BQ-250-230 quad breaker. (Eaton Cutler-Hammer makes them now, find a local C-H supplier.) Put a small dab of Noalox on the bus before you plug it in as insurance, in case the tin plating is damaged it will seal out the corrosion and prevent arcing - you hope...
That gives you a 50A 220V pair on the inside for the welder, and a
30A 220V with the big hoop handle-tie for the water heater. Install your welder receptacle "temporarily" on the wall (so you can remove it quickly with a few screws and spackle the holes) and you're done.And that's going to make that panel just about full load-wise - don't try running everything at once, or you'll be tripping the Main Breaker and dealing with an annoyed SWMBO who doesn't like the unscheduled blackouts.
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