I've had good luck in cold weather keeping my feet warm by using:
-Pearl Izumi Elite Barrier MTB Shoe Covers (neoprene coated with waterproof PVC)
-Wool socks
-Hot Hands feet warmers (cheap - 1 pair lasts 4-6 hours)
Definitely switch to clipless pedals -they really improve performance - which is important if you are riding 400k.