I think you would be wise to mike the threads from the original nipples and get an accurate diamter. Also check the thread pitch with a thread guage. Threads were no standardized until roughly 1900 or a little later. Also in my experience it is not uncommon to find threads that have been worn or damaged and chased out to a large diameter of the same pitch. Further you need to confirm that the cylinder threads are still intact and do no need some overhaul to freshen them up.
It would not surprise me if you needed to use custom nipples made on a lathe.
cheers mooncoon