With headers, I think all you'd need to do is add an X-pipe, good filter and program the computer a little. After that, just smooth out the air coming into the engine and maybe a slightly larger throttle body and you'll be at 400 without breaking into the engine.
To get well over that and still not fool with the engine, you can always think about the light flywheel, clutch and pressure plate. Not only will you get more to the ground, but it will spin up much faster. We can't do that in T1, but I've seen plenty of road cars tested/dyno'd and they all worked very, very well. I'd leave the engine alone - you don't need to touch it to get to that 400 mark.
I have a LUK stage II with a Fidanza aluminum flywheel and it's a great kick in the pants... I hit 150 at VIR vs. 145 my last time out before the clutch/flywheel. I'm sure it's a good 15-20 ponies on a dyno.