Building on what Ed said....
Up until a couple years ago, we were running a .100 over 454 truck block (4-bolt) making 1300 crank HP on a medium blown application. This was with a mildly ported GM990 iron head.
The flow, from my understanding, on the 990 heads would have been maxed out with a blower around 1500 HP.
As Ed stated, the GM parts are severely under rated. Great pieces for the money.