RB VS B Blocks

T67POWER said

Demon seed....
I listed the part # for you to read as it says there is a premium cast crank in the four barrel engines.
I have not looked at the crank and how it's machined on my 400 hp engine but since it carry's a different part # i would think it may have something to do with the grade of quality and or how it's been manufactured.

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I have no idea why they would call it "premium" but I doubt it is because of the actual grade of cast iron (carbon content being the big kicker). It could be something as simple as slightly larger oil transfer holes or a wider filet area on the throws and crank journals or even a slight change to the snout. Any change to the original crank will cause a new casting number to be generated which is the norm. Because these cranks were massed produced by casting companies I'd be willing to bet that the same actual cast iron for the premium crank and stock crank came out of the same molten oven that day. Whether the "premium" cast iron crank actually turns out to be more durable or stronger or less prone to harmonics only an engineer of the day would be able to answer that question. But wait for it, there's enough talent on this forum we may have one of these people waiting to jump in on the discussion.