Before doing ANY modifications, evaluate the following:
Is ALL maintenance up to to date?
What are the compression test results?
How is my transmission and clutch looking?
Are my brakes up to the task of handling extra power? (Answer: with stock pads, no.)
Do I have the stock uppipe installed? Read about this topic HERE.
Only after all of that is complete should you move forward.
You can get quite a bit more power by just de-cating it and getting a map done, but If you really want 400HP I would take it to your nearest scooby tuner and ask then for advise on internals, injectors, rails, turbos etc!!!