08 silverado z28

It says the heads are from a 2004 5.3, what I Wana know is if these r any good. What the combustion chamber size. Valve sizes. Any flow data that anyone might have. I have 4.8 heads from a 2008 , how much better would the older heads perform once I get a good port and Polish job on them than the stock heads from an 08? I heard the newer heads were the 243 castings which I also heard were pretty good. Just need some info thanks.