One of the problems with precarve stocks is that they either do limit you because they are closely shaped to final shape OR they do not limit you and are not close to final shape and therefore you have to understand what wood has to "go" to prevent it looking like a club.
Never having used a precarve for a longrifle, so do not know whether there is that much extra wood on a Pecatonica. One may assume they designed it for a specific lock and to be close to profile for that lock.
BUT when building from blanks I let the barrel and lock do the talking as far as any shaping from the lock panels forward. A lock larger in height may necessitate a lower line to the forestock to get the lock panels right. I doubt either of these locks differs enough in the angle of the tail to cause problems like a banana-shaped early lock might.
You've also got to know whether or not you have enough wood on the lock panels. Everything has to be envisioned from the touch-hole.
I would make a tracing of both locks from a catalogue (fullsize pix) and line up the pans correctly then see how much the larger lock extends for and aft, top and bottom, and how that fits into the precarve.
Better yet if you want the full experience start with a blank. You will learn much more.
