[Chambers]

Well, doing a head gasket is not that bad, there are plenty of write ups out there on it and as long as you take your time, do your research, and label everything you take off the engine you should be good.

Ideally you want to get the head rebuilt and planed, that usually consists of the machine shop taking everything apart, cleaning it, and planing.

400hp from a NA is a number that really cant be met easily, the best way to build a engine would be to buy another one. A JDM engine would be the way to go, so you can build it and still drive your car until you finish your engine, you would need to get a stroker and boar kit, then you would need to seriously beef up the valve train, new seals, springs, retainers, valves, etc.. You would want to get some oversized valves as well. Then you would need to get yourself some aggressive cams, but with that will kill your smooth idol and you would have less power in your lower rpm band. Next you would need to have some headers made, as there is no header out there that is correctly tuned for the 7M, then ITB (individual throttle bodies) these would have to be completely custom and would cost a lot, they would also need some math to create the intake stalks as well. After all that you would then have to get a Maft Pro or standalone ECU to be able to run all those upgrades, and then a new clutch and flywheel.

Hope this helps.