Garry,
Somewhat depends on the scale of your engine and the boiler material. For larger engines with steel boilers (if your's falls in that category) then the old standard EN3B or modern equivalent of similar carbon content and UTS is expected for boiler studs as communicated by a commercial boiler inspector. Do not use stainless, generally subject to all sorts of issues such as crevice corrosion (especially if you fill up with chlorinated tap water!) and sudden failure, plus if you ever want to get then out again, you probably won't!
If it's a smaller scale engine with copper boiler then PB is probably as good as anything.
In all cases talk to your boiler inspector. Club inspectors sometimes do not appreciate that the traction engine cylinder and its attachments form part of the pressure system and thus part of the boiler.
Paul.