crush bending ss pipe would be a crime. mandrel bent pipe is the way to go. we do low volume production bending at our shop. even then our mandrel tube bender cost $65,000. even then unless you know the exact degree bend and rotation for every bend, you need to cut bends and straight pipe to make it work. you can't tweak a mandrel bend, once the mandrel is withdrawn the bend is what it is.
your better off buying bends and straight pipe to achieve your goal. a good tig weld will make the bends seamless looking. we do most of our custom exhaust work this way.
409 ss will rust. 304 ss will not.
this is a custom header for a turbo installation. headers would be done the same way. very labor intensive. you need to carefully choose your weld path to make every weld doable.