I wouldn't weld it unless you have too. I think a more elegant (and reversible) solution would be to find the correct diameter of Round Tube to fit the new seat mounting clamp. I think it will fit inside the square hole on the frame. Then mark the holes that line up and drill for bolts to secure it. If it isn't a tight fit, get some flat steel to make shims. The bolts will probably be sufficient to hold you. If there is wiggle room, the bolts may eventually wear at the frame. So make the pipe a tight fit in the frame and the bolts just there to stop it from going up or down. Clamp the seat on the round pipe that sticks above and you're good to go.