X2 and about those ProComp heads what are they. are they the newer 210cc heads or the older 190cc heads. ProComp has had major issues with poor quality in their 190cc heads and they have been flow tested and had about the performance of a set of stock GM vortec heads. i have heard that they are working at getting better quality and i haven't heard anything bad about the 210cc heads. a word of advise would be no matter what runner size they are i would strip them bare, have them checke out and cleaned up and then rebuild them with quality parts as this has been were they have had serious issues.
and issue that just happened with a good friend of mine about 3 months ago was he bought a crate motor with the ProComp 190cc complete heads on it and in less than six months he snaped a rocker stud. and i know this guy he doesn't go around abusing this motor i've never even seen him take it above 3000rpm.
and to be honest getting 500hp out of a 400 by only turing it 5200-5500rpm wont be too easy, possible but not easy, especially with the ProComp heads