I just realized that the Zinsco bus will accept thin 2 pole breakers.
This is a thin two pole 30 that would fit in your panel. The two pole thin 50 would be the same size. You can do what you want because of the Zinsco style bus.
It's still not real safe to pull out old breakers in this style of panel. They do get stuck and often have to be coaxed out.