Use as large a cup as you can manage to keep the shielding over the cooling weld puddle. Keeps the amps low to control your travel speed, too. How thick is the tubing? Are you going to make this two piece, or all one piece? Be sure you have excellent fitup and unless you know the exact alloys, use 309L for filler as it will handle a wide range of alloys, even if they are not the same.
I bought an exhaust system off another bike expecting stainless, but got titanium instead. Something to watch out for.
Long arc, short arc, heliarc and in-the-dark!