I recommended the 309H because it is rather economical, has sufficient strength at elevated tempertaures, has sufficient ferrite to provide resistance to hot cracking, and is quite easy to weld when joining it to carbon steel. If you are not familiar with welding stainless steel, then stick electrode might give you a little problem due to the fluidity of the stainless steel rod. If you are familiar with TIG welding, that will make it quite easy for you. You should have no problems. You will need to set the heat to whatever weld process you choose to use and what thickness the headers and pipes are. Give it a shot, you can do it...
Chuck