Lifted from my other favorite website
http://www.mig-welding.co.uk/ well worth a look
Ideally to weld mild to stainless you'd want a stainless wire and very low CO2 content gas. Mild steel wire would reduce the corrosion life, and CO2 gas might make the join a bit brittle which could possibly reduce fatigue life. If I were doing the job myself I'd use the stainless wire that I have which is close enough, and argoshield light gas which is close(ish) too. Of course a proper welder would find out exactly the right combination.
The sills Lee 944 mentions are very well made, but miss the end profile on to the rear arch-nothing that cant be fixed with a bit of bending, and also the all important drain holes - again, mr drill comes in handy.
Brian