Yes they were made there, but according to

http://en.wikipedia.org/wiki/Ford_335_engine#400

"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."

which is in line with what I've heard for decades now...

And here is some more info on performance versions;

http://www.projectbronco.com/Technic...e_build_up.htm