[JLMounce]

For a given weight, drag and gearing, the amount of power to sustain driving costs roughly the same, regardless of how much power the engine itself makes.

If nothing else is changing and you aren't putting a 400 in it that is more radical than your 350, it's unlikely that you'll see less fuel mileage. If the 400 makes more power and you find yourself accelerating harder more often to use it, that's where a fuel mileage decrease would come from.