Horse Power Potential of Cylinder Heads

I'm building a 340 stroker motor for my Dart sport. What is the relation ship between head flow and power output? Is there a simple formula to figure out how much flow you need to produce the power you want? If I use my small valve stock J heads what is the most power I can expect? E brock rpm heads? My goal is to make as much power and torque as possible with a max rpm of 6000 or so.