FMX

351C comes with the same smaller bell pattern as the 289, 302 / 5.0 and 351W. the 400 comes with the same larger bell pattern as the 429 and 460, unless you happen to have one of the very hard to find 'FMX 400' blocks that came with a small bell pattern cast into the block and machined to accept a small bell pattern transmission. the good news is the FMX transmission does use a separate bell so you can change it for use on the large bell pattern engines. the bad news is any FMX bell housings may be getting hard to find?

here you can see the unmachined large bell pattern and the machines small bell pattern of the FMX 400 block

none of the holes or alignment pins coincide, word is that there's not enough meat in the casting to drill / tap the large pattern into the small pattern block i'm not sure if the large pattern block can be converted to the small pattern. if the engine you're getting has a large pattern FMX bell on it then you should be able to swap that bell onto your transmission but i can't guarantee the torque converters are the same, and i'll say the flex plates are probably different but swappable. starters i also do not know 

Thank you for all your replies! I'm buying the motor/trans tomorrow, thanks to this info!