You really need to get the Ford Falcon Performance Handbook from Classics Inline. Click on the 'Performance' link at the top of this page, then click on 'Classics Inline' and order it from there. The handbook is pretty much the bible of what we do here and It will tell you everything you need to know about all these things.
However, in short, the head for the 250 is a later development. It has larger intake/exhaust valves, a larger log and larger carb throat for better aspiration. It does not increase the number of cubes in your plant.