Hey stueyI'd recommend a pipe expander tool. You can get them for about $15-20, and there are varied sizes.
They are great as you can flare one end you are joining to allow the other to cup into it which makes a fillet type weld in place of a butt weld. May even help the flow reducing eddy currents and less internal penatration. Maybe google or you tube it. I bought one half way through fitting my exhaust, mandrel bends etc. and it made the job so much easier.
Additionally, I'm told if you use the right gas you can weld stainless with a mig!