Hi Mark
JIS G3101 SS400 is equivalent to ASTM A36 steel, so anything the code recommends / requires for A36 (Category B) will also be applicable for SS400. This means pre-heats of 65°C are required once you go above 65mm, and increase in temperature the thicker you go. (I am assuming that you are using AWS D1.1 code, based on other posts.) Just make sure that everyone is happy with the way that the "Unlisted" material is being handled from a code point of view. Different clients / inspectors / engineers will deal with this differently. (In terms of WPS requirements etc.) Some will deal with it as being equivalent to A36, while other will insist that it is dealt with as an unlisted material requiring a lot more testing etc. - Best to get an agreement on this up front, to prevent any surprises later on.
Hope this helps.
Regards
Niekie