I just scanned/searched both preprints and was unable to ascertain the Reactor alloy. Definitely stainless steel.
Likely 304 or 316.
I don't know what type of stainless steel it is. Here is what I do know: you must test the reactor it without Ni reactant in it, to be sure it does not release contamination at high temperatures. I suppose either type of stainless steel would be okay if they pass a high temperature bake out test. I suggest you ask an expert to make the reactor, and specify that it must be leak proof at 0.01 Pa and it must not release any contamination at 400°C. I do not think an ordinary person is capable of fabricating something like that, but I wouldn't know. I rely on experts for things like this. So does Mizuno.
I would definitely not try to use something like a stainless steel truck exhaust pipe, the way someone here suggested. I suppose someone would do that to save money on the experiment. I do not think it is a good idea to economize when you will probably have to spend weeks or months working on this experiment. Suppose after months of effort you find out that a truck exhaust pipe contaminates the gas and ruins the experiment. You will have wasted all that time and effort because you did not spend an extra few thousand dollars in the first place. This is folly.