The Ford 400 engine was based on the 351 Cleveland but was produced with a half inch taller deck height to allow for a crankshaft with a longer stroke. The 400 also featured larger main-bearing journals and had "square" proportions, with a 4.0 in (102 mm) bore and stroke; it therefore displaced 402 cu in (6.6 L), making it the largest small-block V8 made at that time. It was introduced in model year 1971 with a full half-inch (12.7 mm) longer stroke than the 351 Cleveland, making it the longest-stroke Ford pushrod V8 engine. A long-stroke engine has good low-end torque. This was a good compromise given Ford's requirement for an engine to power heavier mid-size and full-size cars and light trucks. The M-block, as it later became known, was the last pushrod V8 block designed by Ford. The M-block also shares some elements with the Windsor engine family: bore spacing, cylinder head bolt-patterns and crankshaft journal dimensions
The 351M designation is commonly referred to “Modified”, and is derived from the use of both "Cleveland" (block, heads) and "Windsor" (crankshaft) components in the same engine. A "Modification" for the parts intended application so to speak.
And if you try to use the 351m crank in a 351w you would need to machine down the front of the crank so the 351w timing chain would fit.