Big Block and small block are more of a chevy and dodge thing than a ford. We do refer to the 385 series and FE series as big blocks. The 220 - 351W blocks are considered small blocks. The 335 series is highly debated. Cubic inches or wall thickness has very little to do with it. Ford makes improved 351W blocks that will take a larger bore than my 390. Ford made many different V8 blocks and chevy only made 2 different styles. If you are racing your ford in a small block only event you will probably need a windsor series engine. The cleveland might also be accepted.