Bill Beyer, moderator of the 335 series forum (351C, 351M/400) posted the following in the 335 series engine forum guidelines:

"Avoid speculation on whether the 351M/400 is a "big block" or "small block". These are generic terms which generally apply to Ch*vy motors not our beloved blue oval mills so they really have no place in this group. The 351M/400 has elements relating to both the 385 series and the Windsor series and attempting to categorize them is futile, especially when using monikers which FoMoCo never adopted."

FWIW, the bellhousing bolt pattern on the 351M is the same as the 429/460.