To avoid scorching the stock, test the pewter with a toothpick before you pour it.
If you can hold a toothpick in the melted pewter for a couple seconds without burning it, you are good to go. The toothpick will turn a little bit brown, that's okay. Dark, scorched/burned toothpick is not good.
Also, plug your ramrod entry hole with a dowel wrapped in tin foil. You do not want any spillage to go down the ramrod channel.
You need to make a dam around the area to be poured. Like mentioned before, use Manila folder cardstock wrapped around about twice. Blue painter's tape works well. Make sure you don't have any gaps for the pewter to run out.
Some guys heat the barrel to help keep the pewter from solidifying too fast, which causes voids or missed spots.
Pour it with the barrel in. If it is a full stock, be sure to plug the barrel!!!!
the pewter cap doesn't need to be super thick. Thinner is easier, but too thin and the pewter won't run all the way down because it will cool too quickly.
Probably about as thick as a brass nose cap is good. Maybe .050 or so.
Also, drill two holes through the wood into the barrel channel, maybe 1/4 inch holes or so, and then countersink the hole inside the barrel channel. This will fill with pewter and act as a rivet to hold the nosecap on.
Here is a link to a post I did showing my first pewter nose cap, mistakes and all:
http://www.muzzleloadingforum.com/fusionbb/showtopic.php?tid/297544/post/1526149/hl//fromsearch/1/
Cheers,
Chowmi.