Just look at what you've started Cobalt327!
(Just teasin' )
Yep, the Ford 400 came out in 1971 passenger cars and was later installed in 1977-82 light trucks.
The last year for the 351 Cleveland was 1974, and it was commonly found in the Torino, Cougar, Thunderchicken platforms.
The 351M was introduced in 1975 -78 passenger cars, and later installed in 1977-82 light trucks.
Wikipedia 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.
It was also tagged with the "M" in order to differentiate it between the "C" and the "W".
I know, I know ... it's just about as confusing as a Chevy 383!