Actually maybe what would be the best is run a root with a 6011 cause its gonna penitrate more than anything then fill with the mig cause i looked up a 120 hobart mig in my northern and it says it welds 3/16 with flux core and 1/8 with solid plus sheilding gas so i would only use that for fill and up. And you can use a 7018 for the cap and that would work real good. I personally would arc the whole thing but I just love to arc weld a lot. lol.
O another tip if you use the mig maybe use .023" wire cause you can use that on its highest setting and go real slow with a slow wire speed and it will have a lot longer time melting it all then more penitration. One of my friends told me thats what they do for welding 1/2" hardened steel for his dads snow plows so it might work good for you. Just would have to go really slow and watch that puddle.