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I suggest you look through this forum for other threads regarding your question.
This topic has caused many flare-ups with people and as a result the thread was locked.
The popular thoughts are
Modified
Michigan
M because the other letters for the 351 where C & W and the M fits somewhere in between.
This really is a can of worms.
Consider the M to be anything you want it to be.
Because it's just a letter.
Here are two links to this topic.
Afetr reading both of these you will probably understand this more.
It doesn't really matter what the M stands for as long as you don't call the 400 "M". There is no 400M, Ford never called it that and since there was only 1 400 it never needed a designator.
I've read that the "M" stands for Midland, the engine assembly plant; but then those same folks (magazines mostly) call the 400 an "M" and it really never was called that by Ford. Around here you'll hear the "M" called "modified"; used as a reference to it being a taller deck 351C-2V block with bigger main bearings.
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