Junkyard 5.9L Magnum + Edelbrock LA heads build

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Except the 400 4speed crank is forged where the 360 is not.

Were there even that many made though? And I don't think they kept it forged for very long into the 70s as the power went down.

So what rings should I look at? I'm guessing i'll have to file-fit them myself or do they come pre-gapped in different sizes? Is there a gap spec for factory Magnum pistons when adding light boost?

Also i'm guessing when I remove the crank to replace the bearings the rear main seal will be "disturbed" and i'll need to install a new one?