One thing you can't modify is the location of the ramrod hole. Having this located deep relative to the barrel limits how much can be removed from the bottom of the stock below the lock panels to give the gun a graceful contour. I would recommend drawing the location of the lock and the final contour you are looking for and seeing how these will relate to one another. As you are aware, it's lots more work to set the barrel deeper than to set the ramrod groove deeper, and I would avoid doing so unless absolutely essential to your rifle. On the subject of where to set the breech, you need to know where the touchhole will be to determine lock placement, but as the lock's not inlet, you have some latitude. I would just cut the breech end of the mortise square to accept the barrel flush at the back, and then inlet the lock to relate properly to the touchhole. Hope this helps.
