304 Is stainless... Its a number which refers to its metallurgical composition. 304 is your general purpose Austenitic stuff you see everywhere for food grade places followed by 316 which is a bit better. The best grade to use for Exhaust is 409, a Ferritic stainless with no nickel content, instead it is stabilised using titanium.
I think the problem with the chinese stuff is that although they may say its 304, its either full of impurities or its composition is only loosely based on the proper ratio of Carbon, Chromium and Nickel to make it. Personally I'd suggest using Mild steel for all the flanges or machined surfaces if you're going to use 304 to avoid warping.
Sorry to rant, I use stainless for everything where I work, so thought it should be cleared up.