While I am not trying to interfere with the hair splitting (mine has become too thin ;-) ), in case others are wondering why a 391 crank the reason it...
390 crank = cast iron
391 crank = forged steel

As to whether or not a forged steel crank is needed in most 390 build ups may be yet another matter, but I would think that it would come with some bragging rights.

Bob

 

Flash,
The reason for the small amount of bearing size difference is for different clearance specifications between the two engines. The actual diameter of the journals on the crank are exactly the same size.