Technically the 351M/400 is a small block (335 series like the 351 Cleveland)it shares the same bore spacing, cam to crank centerlines etc. It is however massive by comparison and some do refer to it as a big block, probably because it shares the bellhousing pattern and motor mount bolt pattern with the 385 series (429/460). The 1973 400s installed in big Fords however had a regular smallblock pattern. Since dimensional parameters are what puts an engine in whatever class the 351M/400 is really a large smallblock.
[ November 03, 2002: Message edited by: woodz428 ]</p>