Did you put in insoles? Some people don't know that if you put in insoles you have to take out the ones that come in the boot or else it gets really tight. Boots do need a bit of breaking in as well, but they shouldn't be killing your feet like you are describing. Are they too small lengh or width wise? Talk to the people where you bought them and tell them your problem. Also, SPK liners can be custom fitted I'm pretty sure by whoever you bought them from. Talk to them I'm sure they can help. Good luck.
make sure your boot model has custom liners, and the shop has the technology to do so first. (Just call and ask) Also, try putting on your boots for 20 minutes now, and if your feet get numb then come back here and tell me.