C&H Die has the dies but they are pricey ($78.25) and Buffalo Arms carries brass made from 25/06 cases at $34.00 per twenty.
As to safe reloading, I've not seen any current data so its kinda feel your way. Best of luck with this fine old soldier
Welcome to CH Tool & Die / 4D Custom Die
6mm Lee Navy Cases-Obsolete Cases-Obsolete Brass
Art
PS... by the way Phil at S&S is a top flight guy. You can't go wrong there. Also, Check out the premier book on the rifle "The Winchester-Lee Rifle" by Eugene Myszkowski (GeneM) another great guy and a wonderful source of information.