Getting to 500hp is going to be difficult with the stock heads. I agee that you would be better to buy Eddy's or another modern aluminum head and start there.

To attain 500 hp you will need to probably still put some port work into those Eddy's to get the flow numbers up. An altenative might be to bore and stroke the 440 to a 470, 500, or higher. That will give you a better compression and more inches to get you there!