Differences between 2nd and 3rd gen blocks?

[Fieroman]

Specifically the 3.1. How different are they? Can the heads interchange?

EDIT: Ok, so yes, the heads do interchange.

[Guest]

There are so many differences it's easier to list what isn't different:

The bank angle, cylinder spacing, the head bolt pattern, the trans bolt pattern and the front cover pattern (though some of the bolts changed size).