well i know Scott Johnston AKA the madd porter recently done a little test between heads i believe the difference between his ported early heads, D0VE, C8VE, etc., was only around 30 cfm. now bear in mind i think he was running non ported aluminum heads vs. his modified iron. so with that in mind the only reason for switching is to get the last couple ponies or for detonation. by the way for those who don't know Scott is the owner of reincarnation the link i gave in a previous post.
450-500 ponies is easy to achieve. a good port job on at least the exhaust side. zero deck the block, bore .030 over,a set of KB 22cc D dish hypers, a good set of connecting rods i personally would suggest the eagle h-beams, either a stealth or an air gap, 750+ cfm carb i wouldn't go much bigger than 850 no need, select a cam for your rmp range i myself would call comp or get with scott and have one custom ground,a set of headers wouldn't hurt but you will only loose about 20 hp if you decide that you want to use your manifolds.
also ya might want to do some talking to paul kane about the oiling mods and oil pump he sure knows that part better than anyone i've talked to.
this combo should have ya knockin on 500 hp's door. set it in and hold on and have fun.