I assume you are using a wire fed welder, slow the wire speed down and decrease the heat and do little spots an inch or two apart, then after it cools some go back and fill in. If you got a piece of copper, put it behind the crack and keep it up with your welder to help with the blow through. A piece of copper pipe pounded flat works good. You might need a helper to hold it there for you, hope you have two helmets.
Solder won't stick and will melt away and fall out faster than you can put it in.
Torch and brass (brazing) would be a better option but that is almost as hard as soldering.
