I have a 351-M in my 1953 Ford. I can't speak for the C's but the M's have a history of bolts backing off. Mine so far 1: an Intake rocker bolt backing out far enough for the push rod to pop out of the rocker. It actually sucked oil from the valley area past stock steel 1 piece intake gasket into the cylinder, ran on 7 and smoked like crazy. 2: The bolt that holds the fuel pump cam and cam timing gear backed out, There's about 1/2" clearance between the pump cam and front cover plate, so the bolt and cam couldn't fall off. But they made enough of a racket that I nearly went nuts trouble shooting the noise.
It needed a new timing chain set anyway..
