The 400 is a great platform to build big strokers out of, but you can still make some power in its stock form as well.
As stated you want some more compression. One way to do this would be to find a set of closed chamber heads. 915's would be prefered but the 516's will work too. Have a good valve job done with some oversize valves possibly some bowl work and you'll make some decent power on the cheap. A mild cam and a dual plane intake would round out the package nicely.
Ted
