263 is still a Buick, it's the shorter straight 8 family. the 263 was actually the last straight 8 Buick ever had in production and in 1953 shared production with the first year of the Nailhead.
The box doesn't say anything about the rings being for a Buick. I was allowing for the possibility that they might be for a different 3-3/16" engine, like Ford, for example.