In addition to the good advice offered above, you need to define where you want the 400HP (wheels or crank) and there is a big difference between 350 and 400. HP and speed falls under the laws of diminishing returns: the first extra 100 HP is a lot cheaper than the next 100 HP, which is a bargin compared to the 100 HP after that.
How do you expect to use the power, how reliable do you want it to be, and how deep are your pockets? Remember the rule: Cheap, Fast, Reliable... pick any two.
What else are you prepared to do to make the car handle that kind of power? Unless you are planning and budgeting major upgrades to the suspension, drive line, tires, and brakes you'll never put the power down.
Is this (by any chance) your first car?