I've done a bunch of these in the past. I make a wood cover for the top of the barrel ( if you have a piece of the forestock that was pre-inletted for the barrel, and needed to be cut off, that will be perfect). This caps the barrel, and stops the pewter from escaping. By the way I almost never use pewter. I use pure tin if I can get it. Tin makes a cleaner casting with far less air bubbles. As for the hole for the ramrod, I use a short piece of dowel that is larger diameter than the ramrod hole, carefully tapered to make a tight fit. If the metal leaks into the ramrod hole you have a real problem. I wrap the outside of the stock with regular old file cards held in place with a couple pieces of light gauge binding wire wound up pretty tight. I fill any small gaps between the barrel cap and the forearm with regular old modeling clay. Rubbing the wood that is captured under the new nose cap with a wolf carpenters pencil will make the metal flow over the wood better. Just before you start your pour you need to drop a long, red hot, bolt down the barrel, to heat it, and keep the metal from freezing. I use a brass carriage bolt with a large head, so it doesn't go all the way down the barrel. Its not as hard as it sounds. I did my first one on an old CVA pistol I got somewhere that didn't have a nose cap, and it came out fine.
Hungry Horse