One problem that I know of that the 400 SB (I'm assuming you are talking 
about the 400cid small block) vs. the 350 SB is that the 400 has an internal 
heat problem because the two inside cylendars share walls with the two 
outside cylendars walls, this was the only way they could make the 400 and 
still keep it the same size block as the rest of the small block family. 
Parts interchangability is another problem, the 400 uses it's own heads,
heads from the rest of the small blocks (350 on down) have to be modified 
before they can be put on the 400. There are some other parts that cannot be 
swapped as well but I can't think of what they are right now. As far as I 
know those are the only problems.
The rest as you said "One spins faster, one makes more torque" are a matter 
of personal taste and/or what kind or work you want the vehicle to do.
-Landon
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