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I would bring the full zip rain pants and heavy fleece pants to put on under them. Full zip fleece may not be quite as warm but the ability to put them on and remove them with your boots on is worth a lot. That's the way I go. If your shells are large enough you won't have to duplicate anything. Once in camp nothing beats down. Down pants are worth their weight in gold when its really cold, and a pair of mukluks is good. My mukluks where made from cheap x large booties from campmore with gaiter tops attached and sealed with seam grip. Don't forget gloves or better yet mitts, a balaclava, and extra large sleeping socks, this is the one place where I carry wool - sleeping socks large enough to not constrict blood flow. Also carry at least a foam sit pad, without it sitting out a cold night is very uncomfortable.
Jim