351m/400 Small Block Or Big ?

[~The Jester~]

Neither. The 351M and 400 engines are nominally Cleveland-style small-blocks with higher decks. But the “Ms” also have a 429/460 rear block-face bolt pattern and unique engine mounts not shared by other Ford engine families. All small-blocks are externally balanced, so be sure to use the correct harmonic balancer and flexplate/flywheel for the application in question: Pre-’81 302 (and smaller) engines and all 351 engines require a flywheel or flexplate with 28.2-ounce unbalance; ’81-and-up 302 engines require a flywheel or flexplate with 50.0-ounce unbalance.