Hi,
This is really not difficult. As other wrote, simply install the rib and pipes on the barrel, place the barrel in the stock and use the pipes as your guide. This also insures that the hole is lined up with your rib and no unsightly gaps will result. However, if your barrel is tapered or swamped, you may have to taper the rib to keep the groove and hole parallel with the bore or allow the groove and hole to be parallel with the profile of the barrel. I choose the latter although is does mean still have to taper the rid for any flare at the muzzle, but I let it follow the profile of the barrel from the waist back to the breech. This enables a nice thin but even web of wood for the barrels length rather than the web thinning toward the breech. It makes it easier to fit barrel tennons and the forward lock bolt if there is to be one. In my design, I also keep a constant thickness of wood below the ramrod hole, which automatically gives the bottom of the stock a pleasing taper from the trigger guard to the rear ramrod pipe. That gives the stock a robust, business-like but elegant appearance. Also, I urge you to inlet your pipes into the rib. Don't just solder them on top of the groove.
dave