If you are determined to use Stainless steel, 321 is a good choise, as it is readily available and is generally fairly cheap (for stainless)
Cranking and fracture of a manifold is 99% due to improper weld procedure (pre and post heat treating not done) or improper welding gas mixture or weld rod selection, or a combination. If you have a SS manifold or header made, the builder must have the proper equipment, gas mixture, and heat treat ovens to properly weld it. Often times the design will not account for the thermal expansion of SS, a disadvantage of SS. SS has a very high coefficient of thermal expansion, in other words it may expand 2-3 times what a normal iron or steel alloy or carbon steel would expand when exposed to the same temperatures. Your design MUST account for this, or otherwise you end up overstressing flange bolts and/or weld seems and ultimately fracturing them.
I would recomend sticking with steels for that very reason. . . thermal expansion is not a big issue with steels.
Enjoy,