I have wondered how the smokebox front ring was sealed off. I would assume that asbestos was historically use as a gasket material for this joint. What material will you be using now to make the new gasket?
Oh yes. You can be sure that the traditional material was good ole asbestos!. I can remember volunteering on UK railways in the late 60's / early 70's. You just pulled reels of asbestos rope or tape from the stores, cut lengths to fit around the plate and tightened up the bolts. No alternative is anything like as simple and effective.
We have sections of a high temperature gasket material that we are cutting to fit the face plate. Much more difficult as it is thinner and must be carefully cut to fit all the bolts. Much more work than just winding the ropes around between the bolts !