309L type filler metal is the usual choice for joining 304 and 316 to mild steel (mild steel with a little bit of copper in the case of cor-ten). It is used because if you use a mild steel filler, or a 308 filler, the weld will have martensite in it, which can lead to cracking due to brittleness. I would also reccomend using 304L for your base material, as 800F is right on the edge of the carbide precipitation range (sensitization), which leads to depletion of chromium and accelerated corrosion if there is enough carbon in it.