Here's some more ammo (though not Factory Ford, which my first link was):
Ford Small Block Discussion
Look at table of contents here (Factory Ford documentation):
Info:
Things in common with 289/302:
cylinder heads will interchange
bore spacing is the same
289/302 bell housing bolt pattern
carries the same 28.5 in/oz imbalance engine balancing
Flywheels interchange
All this 351 small block/big block came about describing the difference between a 351W and 351C. In reality, both are small blocks.