I think a 400 is just a bored out 389 and has 3.00" mains.
The 421 and 455 have 3.25" mains.
Some what they call, "Super-Duty" engines have forged cranks whereas most others have cast.
I forget if Pontiac ever ran cast ARMA-STEEL rods or not??
Not much help..
pdq67
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