I have built one Hawken with a tang shaped like the one in Chucks photo and one more extreme. Almost a reflection of of the fence on the lock side.. Your breech is just not shaped right to get either one.. It looks completely wrong to me, even for a standard tang. May be the angle of the photo but I don't see how you could inlet it into the wood ? I would give some though to a completely new breech. Might save a bunch of problems in the long run ,,,Louie