welding cast anything is tricky,
i'm assuming your migwelder runs on 110, if so here's what i would do , get the surfaces your gonna weld together super clean ,grind off all the rust, paint, oil whatever. crank up that little 110 as far as it goes . and stitch weld it , make sure you go real slow, and if it bubbles up with contamination, just wait a second till it solidifies , then either just keep going or grind off the bubble if you want it pretty
i think this would hold just fine for a mailbox , no worries at all fuzz