Some good tips listed here, a few more:
351M-400 have a flat thin steel timing cover between the water pump and block, 429-460 has an cast aluminum cover, approx 1 1/2" thick.
On a 351M-400, the fuel pump bolts to the block with a vertical 2 bolt pattern, a 429-460 fuel pump is attatched to the aluminum timing cover, with a horizontal 2 bolt pattern.
From the factory, ALL 351M-400s had 2 barrel carbs, all carburated 460`s were 4 barrel, although some early 70-72(?) 429`s were available with a 2 barrel.