seems like I opened a can of worms here...

DD,
So if its is the revs that warrent it, then I would have to say that the Revs are the symptom, rather than the cause.

Would it be that standard cranks are not always well balanced, and that high revs in larger capacity, bigger cranks would cause more siginificant vibration and flex than a smaller crank, in something like the a series.

Dman,
you speak about it as though its got to do with the load placed on the engine rather than the revs??
Since I would expect trucks motors to be a low reving unit, at constant-ish revs, whereas a passenger unit which may rev higher, and a more varied rev pattern.

The web pages that i have looked at all say that 4 bolt mains are better cause its stronger, and should be considered in any high performance engine. We all know that. The reason for it still seems to be a bit vague.