500 HP is not really a problem. GM made lots of good flowing heads for BB's, but what are you going to do with the rest of the combo?? Depends on which piston you want to use. 049's or 781's (last 3 digits) are preferred open chamber oval port heads, but fairly large chambers so you need a domed piston to get a decent compression ratio. 215's are a good closed chamber head. They have small chambers, and you can get a good C/R with flat top pistons. There are others, but these come to mind right away. You pretty much need to figure out the rest of your combo at the same time, then you can pick the heads. Oval ports are the way to go if it's primarily a street engine and you want good low end torque.