Obviously they are both true. If they are both false, that means neither box contains the $1 Million and you are being scammed and will die. So they have to both be true.
Now, you know A and B are true. B says the snake is in Box A. By process of elimination, you can be sure the money is in box B.
If this was real life though, I still wouldn't open either box and just accept getting no money lol