How about 1/8" SS and then weld on the cut-off tiangles as stiffeners from bolt holes to exhaust hole?

If I'm not mistaken, Tracy said he made some flanges out of 1/8" steel (don't recall if it was stainless), and I guess they worked.  Seems like he felt that was pretty minimal though.  I don't think I'd hesitate to use 3/16".  Personally, I'd probably rather use stiffer material, than go to the trouble of welding the stiffeners on.  

Rusty;

It’s not the stiffness of the flange we’re after; it’s strengthening of the pipe to flange joint.  It’s an area of lots of thermal stresses; and the flange to pipe weld is likely the most common failure point, especially if there is not flexibility for stress relief downstream on the pipes.  Of course the geometry and the welding of the stiffeners is a bit critical as well.

Al