The higher the number generally speaking the higher the temperature and corrosion resistance due to a higher nickel content. These properties come at a price, reduced tensile strength! The 304 has good tensile and other mechanical properties, if I was making a SS chassis that would be my choice. Yarde deals in alloys, if you are looking for 1020 DOM (which is what my frame is) you are talking mechanical tubing. Good place for price and availability on that is
marmon