I cant remember what the shape of the water ports are, but it should be clear if you lay the gasket over the block and compare against the head. I remember looking at them and wondering if I could cut out the square to make the port rectangular.
Maybe the gasket is blocking off some of the key waterports in the old head resulting in a high pressure area that pushes out.
I'm pretty sure they must be quite different though because otherwise I probably would have tried to use them as well :D