As in building from a blank, I would recommend that you lay the gun out on the stock as best you can. Don't know how much wood is left to remove, if you have flat surfaces on the sides. But laying the gun out will show you where the lines of the finished gun will be. Lay everything out. Thimble spacing, barrel pin locations, butt plate, lock, sear and trigger locations. That way you can see how everything fits together and lays out on the stock, and you'll have lines to follow when it's time to shape.
With a pre-carve I imagine there is plenty of wood left in the wrist and butt; so that lowering the barrel inlet will also allow for the proper architecture in those areas.
Clamp the barrel in place, centered on top of whatever barrel inlet is there, and draw lines to follow in inletting the barrel. Also, I would recommend removing the breech plug to inlet the barrel, then installing the breech plug to inlet it.
I like to lay out for a web thickness of 3/16th at the muzzle, and 1/8 at the breech. You can do that with the pre-carve... just sink the barrel down a 16th of an inch more at the breech. 1/16th isn't much, but it does make a difference in how slender the gun looks, and feels.