Hello,
Welding 304 or other 300 series stainless to mild steels such as A36 is pretty common.
there are many sources for this. the Most common Filler metals for this combination of metals is either 308 0r 309 Rods or electrodes.
Good resources for information are: AWS D1 series code books, Lincolns Arc Welding Procedures book. The AWS welding Handbooks. AWS members receive a copy of the Welding Journal. over the years, the subject has been addressed.
Many Local libraries have Technical books that will help you out.