My uncle has a 3/4 ton 1982 Ford w/service body. It has a factory 400 w/C-6. Chiltons lists it up to 1982.
The 400 was introduced several years before the 351M. You are right the M is to seperate it from the other (C & W) blocks that Ford produced with the 351ci displacement. The 400 is the only 400 Ford every produced and does not need a letter after it. Ford was ending production of the 390 and need an engine with the same displacement for trucks. The deck height on the 351c block was changed along with a few other small details (main bearings) and it was stroked to 400ci. Since the 400 is an offspring of the 351C, then if anything it should have a "C" after it. I know of several books that call it an "M" block and refer to it as a 400M, but Ford never did. I believe even Chiltons puts a "M" on it.