In the 60s, car manufacturers sometimes assigned displacement numbers for advertising purposes or to differentiate their engines from their competitors. Ford/Mercury and Pontiac were the top offenders. Check this out: Advertised Displacement Actual Displacement Ford 427, 425.816 Pontiac 428, 426.613 Ford 428, 427.386 Pontiac 350, 353.42 Pontiac 326, 336 5 famous V-8s whose displacements stretched the truth - Hagerty Media
It's only a couple of Cubic inch either way, nothing that bothers me, like a fractional number on the air cleaner would.
In the 60s/70s, car manufacturers intentionally under-rated the horsepower of their performance engines. This was done for a competitive edge at the track and to minimize insurance surcharges. For example, the 340 high-compression four-barrel engine was rated at 275 hp. However, the NHRA factored it up to 325 hp.
