Are these letters unknown to the public?
Hmm that`s interesting. Those are letters as the OP mentioned and indeed, I can read that Piłsudski addresses sb as you and talks about Narutowicz.
Letters from that period were published in volume 5 of Pisma Zbiorowe.
So, bronto, have you checked if your letters are already translated? Or do you need your own translation to avoid copyright costs?