All I know is that back in the early 80's when older vehicles were ripe for the choosing for a young person with a few hundred bucks in his pocket, every last one said the same thing, 351M-400 and I never knew what the heck it was. If there was a different engine sticker, I never saw it.
What I did know for sure was they were thirsty sluggish pigs that were fixing to let their innards loose, and even if we knew of all the wonderful fixes for them back then, it would not have been financially feasible at the time, when perfectly good big block engines and transmissions were plentiful and dirt cheap to obtain and easy enough to swap out. Another thought, once converted, the now big block truck had cred where it did not before. All the splaining in the world back then wouldn't have changed a single mind about a truck with a less then desirable engine package in it. Which left all those engines waiting about for today's rodder to mess with and no one thinks any less of it either.
