All FE's are restricted by air flow. You are trying to feed a 482-inch engine with an induction system designed for a 352!
Look at this:
http://users.erols.com/srweiss/tablehdc.htm
Read the explanation at the beginning. People make all kinds of claims about cylinder head air flow. The flow is only valuable with the valve opening within a certain range. It is definitely an art rather than a science. That is why it is better to get a whole package from somebody like KC.
The most you can expect with your late-50's designed engine and still be streetable on pump gas is in the area of 600-650 HP. This translates into about 475-525 at the rear wheel.
There is lots of good advise on this forum about getting that last 50 or so HP. I think you are on the right track with the KC heads. They turn out to be about $50-60 per HP. A ported single-plane manifold will be worth a few more ponies.
Good luck,
Brian