I think you are setting the bar to low. I am in the process of of building a car that will have the capability of 800+FWHP, easily get over 20mpg, and have to pass emmissions.
The answer is simple, turbo. Start with a 76mm garret GT42R good for 1000hp. Make it a 331 with the use of a dart block, and forged internals(I'll be using scat) are essential for the reliability. The AFR 185's fit the bill (for my self as they are emissions legal), and will flow enough right out of the box for even my goals. The 20mpg+ will come via a D&D viperspec T56(required for this level of hp to keep it reliable) and as it has a .5 sixth gear runnig 4.10 gears will still get great highway milage. This combo will require lots of fuel at WOT so a areomotive fuel system with a sumped tank will be needed and a set of high impedance 60lbs injecters are required. This can all be managed with the stock computer and the twEECer or the twEECer RT would be even better. The final and perhaps the most important part is the cam, a stock cam in a combo as described above will likely tap out at the 675-725hp range so i would recomend a custom grind, but keep it small to retain that solid torque down low for the highway crusing
.
But if 400RWHP is what you have your heart set on, the base HP turbo set up with a 60mm turbo and all the parts to make it work as a turn key system makes in that area (433RWHP actually) for around $5000.00. And with a good tune(read consevative A/F ratios) you won't even have to upgrade block or heads or cam, as turbo's don't stress parts like nitros can.
Good luck with your qoals, and remember technology is our friend.
Steve.