Several pairs of spare socks. Good thinking.
In cold weather, wet socks are very bad news. Even simple perspiration will wet them. A lot of people mistakenly think that because it is cold, they don't sweat. That's a quick way to crippled feet.
One thing you should check is the amount of insulation your sleeping bag has around your feet and lower legs. Otherwise you could wind up with frostbite.
A balaclava to protect your head and face might be wise. Having your face inside the bag will lead to a damp sleeping bag. That will drastically reduce it's rating.
I think that you need to develop a through understanding of arctic survival. -40 is not exactly the most forgiving of environments.