My google fu can't find any 17-4 SS tubes and caps. Also what wall thickness would I want for a can that will hold up to FA?

I'm not aware of anyone selling tubes in 17-4. While it is available in tube form, the stock order would, most likely, need to be in the 1000'+ form. That's a big overhead, for a solvent trap supplier.

You can use either Ti or SS(300 series) and use a SS blast spacer, a thicker blast baffle, and a muzzle device, to get it compatible w/ FA use.

ETA: There are some completed builds where the maker used it on FA. Compare your design to theirs.

17-4 round CDS tubing isn't an easily available off-the-shelf item. Your best bet, if you're dead set on using a 17-4 tube/end caps, is to get a local machine shop to bore bar stock out for you and thread it. Same with the end caps, have them make those for you.