Yup, till then, Buick, Olds, Pontiac, Cadillac, all made their own engines in a bewildering array of sizes and configurations, like the Pontiac's 60's OHC six cylinder, Buick had a V6, GMC trucks had weird V6 engines, in the olden days the big cars had straight eights to compete with Ford's Flathead.
Owners sued when they found out the old Buick Nailhead was now a Chevy 350 with a soft cam and leaks from the rear main seals. The other brands were supposed to mean more than Chevy, good enough for work or going to work.
I had a '66 Olds Delta 88, would rattle every window in your house idling in the driveway, as well as interfering with CB and TV ;[] Ultra High Compression 425 Rocket, a tank of gas every night of tooling around looking for trouble and never finding it.