Is there something specific about the 420 number?
The stock motor hardware is pretty tapped out straight from Ford. Reliable power in the range you're talking about requires great expense in the form of a built motor. You might find that you don't enjoy the trade-offs required in terms of noise/harshness of the driving experience, or peaky power, or fuel economy, or you might find that all that power doesn't make this platform more enjoyable.
This is the party-pooper opinion but if you want that much power reliably, start with a different car that comes from the factory with the power you're after. It will be engineered to work well as a system and likely cost you less in the end. Full disclosure - my car is stock, I haven't built any EcoBoost motors, and I am painting with broad strokes based on casual reading of the forum.