at that thickness i would go 100% helium. thats way more aluminum than that welder should be doing. asking it to go into spry is also asking alot, you realy need the MM210 to pull that one off properly but if you must try then 100% helium would be your best bet, get all the heat you can.
if it were me i would farm out the weld part to some one with a good sized TIG unit, figure that expense into the bid you make for the job.