I'd go with 321 stainless.

===> How do you feel about 304 for the outlet of the turbo?  Does the turbo
reduce the heat of the exhaust at all?  Burns Stainless has a nice pre-bent
pipe for my application, but the size is only available in 16 gage (.065")
304.

Turbo installation difficulty is vastly underestimated.

===> I hope you're wrong about this :-)  I'll find out soon enough.

Cheers,
Rusty