yes, it's a 401ci Nailhead, even though GM marketing decreed that midsize cars could not advertise displacement larger than 400 cubic inches so the Nail in the Gran Sport is referred too as a "400" in all of the official literature, even though it's not.
the actual 400ci Buick Big Block debuts in 1967. and the only reason that engine exists is because the "no displacements over 400ci in a mid size car" rule was still in place through 1969 ( even though there were a couple of exceptions permitted like Olds and the Corvette ). when GM dropped the 400 cube rule in 1970, Buick dropped a 455 into all their big block options.
The way to crush the bourgeoisie is to grind them between the millstones of taxation and inflation.
Government schooling is about "the perfect organization of the hive."
H.H. Goddard, Human Efficiency (1920)