There would be no need to ever refer to the 400 as an "M". It was an
engine unto itself. The 351M followed the 400 form.
"M" is an easy way to differentiate the last pushrod V-8 designed by
Ford displacing 352 ci. from the 3 previous versions, & especially from
the 351W that was in concurrent production.
If you tell me your '76 Marquis has a 400M, I'll know exactly what you
have and I won't correct you.
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