Happy New Year John!
I formed the tank using .125 thick 3003 series aluminum sheet and I welded it on a Miller Syncrowave 200 at about 160-190 depending on the section I was welding. The walls or "box" was on the lighter 160amps but moved to 190amps when welding the thick billet cap receiver and the mounting plates because they are .25 thick x 2" wide x 4" long 6061. I also used a "4000" series filler rod *sorry I forget the actually number of the rod* and gas is set at 20lbs. I was using 1/8in. GREEN tungsten with a #6 cup. Hope this helps.
I would use the exact same settings if it was welding 6061 T6 entirely but as you know 6061 does not bend sharp 90 with out cracking. so a 3003 or 5052 aluminum sheet is needed. Or you can anneal the 6061 first with a torch.