The "M" stands for "modified". The 351M is an oddball with parts from all over the place. Let me start off by saying that in 1972 Ford took the 351C engine design and built a new beast called the 400 for us in mid-fullsize Fords and F-100/150 trucks. The block style is the same (timing chain housing, 4" bore,351C 2v heads, etc). The 400, however, used a larger main bearing diameter(same as 351W) and had a taller deck. Here is where the 351M comes in. The 351M was built as a replacement for the 351C even though the engines used the same CID. The 351M used a 400 block(taller deck), 400 rods, 351W crank(3.5"), same heads as 351C 2v/400. The 351M was used heavily in the mid-late 70's to the very early 80's in mid-full size cars and F-100/150. If you look at the engine tag for the 351M it will say "351M/400". I suggest you stay away from this beast unless it was orginally installed in your vehicle.