My new welder should be arriving today (Millermatic 180) and I've got a few questions...

I'll be practicing on some square tubing to get started, but my first real project will be on a trailer I picked up a few weeks ago. The trailer is a Canadian M101 - much like the Bantam and the M416. I plan on cutting in a tail gate and adding some support, and my question is what type of shielding gas and wire should I be using for this kind of work? I used the Welding Calculator and came up with CO2 and .035 ER70S-3 (or ER70S-6)... Does this sound right?

Thanks