The environment is going to be important since you are talking about a pretty a pretty warm temperature. First, the 304 will be ok as long as there are no chlorides. To optimize its performance, bright anneal or electropolish is recommended, but at least passivating should be done. Kovar is not stainless. It has no chromium, so it's no better than carbon steel as far as corrosion resistance goes. I won't recommend an anti-corrosion method since it's probably bonded to a non-metal and I would certainly overlook some complication.

For Kovar you may want to Ni plate. Some here have argued whether electrolytic or electroless Ni is better. If you are only concerned with amient moisture corrosion (e. g., salt fog) then you can cap the nickel with a 40 to 80 micro-inches of electrolytic gold.

This approach for coating stainless will also work, but is not as necessary. Kovar is used only when expansion matching to a low expansion material (e. g., SiO2) is an issue.