I'm just guessing, but I would expect that the 400, being a "stroked" small-block, has to have external balancing out on the flywheel; not enough room in the crankcase for proper counter-weights. So the flywheel wouldn't be interchangeable with another displacement.
Flywheel or a flexplate, F-150 comes with either, new flexplate is only around 30 bucks.
But only can seem to find a 13 buck ring gear for the flywheel.
Replaced the flexplate when I rebuilt my TH-400 mainly to get a new ring gear, but kept the old one for a side by side comparison. Putting the whole thing back together is not the time to find the replacement flexplate has an extra tooth in it or a 1/32" larger in diameter where the starter drive will crash into it.
Could be your terminology is correct and you do have a manual transmission, but even with other engineers, always had a problem with terminology, have to be very specific if you want the correct answer. Even NASA screwed up, one guy was talking inches, other was talking centimeters.
351m and 400m are the same block.both big blocks.they are both balanced the same so balance shouldnt be a problem.i would check to make sure flywheel depth is the same for both flywheels.ford had a habit of making little changes like that back in the day.final authority on this one is your local wrecking yard and their interchange book.
I'm no expert on old Ford V-8s, but the info I read says the 351M and 400M are both "raised deck" versions of the Cleveland small-block (351C). At any rate, they may still use different balancing since the cranks are different.
we allways called them modified or big blocks.the correct term would be modified block.both are raised block versions of the 350 cleveland.351m and 400m are both balanced the same and is far as i remember had identical flywheels.like i said,the autowrecker would have the final say here.