when i worked at ford i was curious about the "m" on the 351m and that is what i was told. i do a little digging and this is what i have found. also i was told the engine mounts are different between the 351m/400 and the 460. seems i was told wrong and gave somewhat wrong info about the 351m name. but they are big blocks, due to the big block tranny bolt pattern
. 351 M
Engine dimensions 351M/400351C Nominal main bearing size 3.000 in (76.2 mm)2.750 in (69.8 mm) Rod length 6.58 in (167.1 mm)5.78 in (146.8 mm) Deck height 10.297 in (261.5 mm)9.206 in (233.8 mm)
When the 351 Cleveland was withdrawn after the end of the 1974 model year, Ford needed another engine in the 351 cubic inch (5.8 L) class, since production of the 351 Windsor was not sufficient and the 390 FE was being retired as well. To replace the 390, Ford took the 400 engine's tall-deck block and de-stroked it with the shorter throw crankshaft from the 351 Windsor, and taller pistons, to produce a 351 cubic inch (5.8 L) engine whose components were largely compatible with the 400. This engine was called the 351M and as a back-formation the taller-deck block became known as the M-block.
The M 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.
Another origin of the M designation may have come from where the engine blocks were cast. It follows the naming convention set forth by the 351C (Cleveland) cast at the Cleveland Foundry and 351W (Windsor) where the majority of the blocks were cast at the Windsor Casting Plant. From the introduction in the Model Year 1975, the 351M engine blocks (which are the same as the 400 engine block) were all produced at the Michigan Casting Center (MCC) in Flat Rock, MI or at the Cleveland Foundry (CF) also known as the Cleveland Casting Plant (CCP). To help distinguish it from the other two different 351s, the logical choice was to use the Michigan Casting Center, hence the “M” designation for 351 Michigan.
The 351 Cleveland had a well known, good reputation in the public. For a couple of years after the introduction of the 351M, Ford marketing called the engine the "351 Cleveland". This led to confusion as to what 351 version was actually in the vehicle.
Later, car enthusiasts incorrectly referred to this engine as a "351 Midland" presumably a reference to Midland, Michigan a city just northwest of Saginaw or reference to an iron foundry in Midland, Texas. But the Ford Motor Company never owned a “Midland” factory.
6. Light truck usage
For the 1977 model year, Ford decided to replace its aging FE big-block 360 and 390 engines in its light truck line with its new 351M and 400 engines. For light truck use, beefed-up blocks were designed. These enhancements were added to all M-block engines starting with the 1978 model year.