Can I add a 50 amp breaker for an arc welder?
Depends on how much load the rest of the house is putting on the service at the same time... but if the total demand stays under what your service is rated for, and you use wiring that can handle the 50A demand, you theoretically should be OK. 

Run the numbers.

(My electric drier plugs into a 220V outlet fed by a pair of 30A breakers, and I have only 100A total service; that's roughly proportional to what you want to do. When I switch to a gas drier, I'm planning to re-route those 30A legs to my workshop.)