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Grind the dirt and paint 1 inch from the crack, with the crack being in the middle, all the way around it, make a clean circle. Then grind the length of the crack , not all the way thru, now practice on some steel that is close to the same thickness as the frame.
Weld the first bead in the crack from one end to the other. Make a 2nd pass on the right hand side of the first weld, overlapping the first weld. Make a 3rd pass on the left hand side of the first weld, overlapping the first weld.
Let it cool, repeat until you are happy.