No personal knowledge - but a Ford forum says-
"The 351M uses the same block and heads as the 400, with a shorter stroke crankshaft (3.50"), the same connecting rods, and a unique piston with a taller compression height to compensate for shorter stroke with the same rods in the same block. In fact, the only functional difference between the 351M and 400 is the crankshaft and pistons. Ford did use slightly different components in the 351M and 400, such as harmonic balancers, camshafts, and valve springs, but functionally, the engines were identical except for the crankshaft and pistons. "