Deck heights are different. early blocks had a taller deck with the taller "domed" pistons and the late blocks have a shorter deck with the flatter style pistons.
This is measuring from the center line of the main journal:
* Early 20R Blocks 75-80 9.120"
* Early 22R Blocks 81-84 9.120"
* Late 22RE Blocks 85-95 8.930"
or measuring from the oil pan rail:
* Early 20R Blocks 75-80 11.280"
* Early 22R Blocks 81-84 11.280"
* Late 22RE Blocks 85-95 11.090"
or this will help :
http://www.lcengineering.com/TechNotes/?techid=4