While Royce is correct let me add that the 351m was brought into being because the 351 was already history and Ford found themselves in a jam because they couldnt produce enough 351's to meet demands. Since they already had the 400 that was introduced to be the sucessor to the FE motors that were having hard times meeting emissions standards of the day they simply destroked the 4" bore 400 to 3.5 to end up with a emissions leagal 351 that could be used in the full size car and truck lines of the day. It was 1977 if I'm not mistaken.
Both the 400 and 351m were never accepted well as they have the same basic block height and size of the 460 and weighed almost as much, with some really dismall HP ratings due to the really low compression ratios, retarted cam timings and such.
They do share the basic 351c designs and the heads are the same as 351 2v heads with maybe a valve size difference. The share the same oil pan, dist and such. The main differences are much larger cranks and journals, the formentioned bellhousing, dedicated motormounts and flywheels and dampners.
They are like Royce said to be not desirable unless you have a original car that had one of these motors and are needed for originality. The good news is since they do share some Cleavland parts there are some things that can be done to help them out, but why anyone would waste there time on them is a good question. Basically there regarded as Fords POS but they did serve there purpose at the time. mm