10-29-2009, 08:19 PM
Both the 400 and 351M use the same casting #'s for the blocks. Therefore externally there is no positive way to distinguish the two. The only difference is the crank and pistons, so to ID you will need to measure the stroke. There is no really accurate way to check it without pulling a head off. Though, when I was looking for a 400 core in the junkyard, I used a zip ty in the spark plug hole. I found an engine that the ty moved more than the others and pulled a head to measure the stroke with a tape measure to confirm.
'72 TBA...