[big gear head]

It has to do with the valve sizes, not the chamber shape. The larger valves in the high performance heads are too close to the cylinder walls and it shrouds the valve on that side. The 4 bolt blocks had reliefs cut into the top of the cylinders for the larger valves. The only issue with the open chamber heads is the way they lower the compression ratio. It will lower the compression more on a 396 or 402 than it will with larger displacement engines.