Depends on how much HP you want? A stock stroke 6 pack 440 can be made to easily put out 500hp. Check out the tech archives, they give the basic rundown of dyno proven 500hp 440's.

Basic combo would be 10:1 compression, good flowing heads(either ported/big valve stockers or eddy rpm/440source stealth aluminums), cam in the ~240@050 range, headers with a good exhaust system, recurved distributor and a well tuned set of carbs.