I have read that you cannot bore a 401 to a 425 .
depends on core shift. the 1/8" or .125" bore difference is a huge overbore for a thinwall casting block but the Nailheads are either before thinwall casting or the first iteration of thinwall that Buick did. they are a bit more robust than the later BBB and SBB designs.
you would still have to have any 401 block sonochecked for cylinder wall thicknesses before you ever even considered doing something like that though.
The way to crush the bourgeoisie is to grind them between the millstones of taxation and inflation.
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H.H. Goddard, Human Efficiency (1920)