Lighter weight, more compact, easier to service and install, can easily make over 500hp, much friendlier to the suspension, better economy, much more horsepower per dollars spent, parts availability, much more responsive throttle to typical driving, not nearly as fussy to tune and maintain, etc... If you can afford it, do an aluminum block and give yourself another 20hp in weight savings.
If the above does not interest you then do a big block. They sound very cool, has the wow factor, has the mystic of originality and not much else.
I built a 393 that delivers 435/444 for approx $6200. You can see my first dyno sheet in the gallery. With a little tinkering it makes more than that now but is classified.
Do it yourself! It was fun and is very satisfying every time you start it.