If you can get a hold of it use pure tung. or Zirconium tungsten. Is the aluminium mill as it does look like it if so you don't need to prep it.Just weld & go with it not that you need it but for a stronger weld I'd go with 5356 wire if you are painting it 4043 will work but if you get it anodized 4043 will turn black at the welds as 5356 will not.

Man thats looking good lot of work but in the end you'll be a proud man.