I´m making me a new exhaust manifold for my 9000 to be able to fit the big GT37 in there. Feel free to take a look at the pics on my site. I will keep puting pics in there this week, will be driving this weekend.
321 and 347 are known as stabilized grades of stainless. These are alloyed with either titanium (321) or columbium (347), both of which have a much stronger affinity for carbon than does chromium at elevated temperatures. This eliminates carbide precipitation leaving the chromium where it belongs for corrosion protection...remember our discussion of intergranular corrosion? Both 321 and 347 are top choices for exhaust headers, especially turbocharger systems and rotary engines. Since 321 is much more available than 347, that leaves 321 as the first choice, with no sacrifice in needed qualities.
316L is an extra low carbon (ELC) grade of stainless that has only .03% carbon, making less carbon available to precipitate with the chromium. It is used extensively in marine exhausts where salt water corrosion mixed with diesel exhaust particulates and electrolysis create such a horrible environment that even other grades of stainless cower and run away!
304 is the most inexpensive and available stainless in the 300 series. It is suitable for normally-aspirated header applications, and has been successfully used by many racing teams. It does not have the high temperature fatigue resistance that 321 does, but is considerably less costly and much more available. Most 304 tubing these days has the dual designation of 304/304L."
Good information there. Since you don't live in the US they would not be practical as a source for metal, but they have a few really great tech articles.
Btw VERY nice job on matching the exhaust ports with the scroll ports on the turbo. The exhaust pulses should be pretty evenly spaced. That's especially helpful in twin scroll turbo cars.