The gm "corporate engine" was begat by the class action lawsuit over- "why is there an olds engine in my ponticac and a buick engine in my oldmobile, that's not what I paid for".
I always thought the furor over that (and there was a furor) was silly as Mopar and fomoco had the same engines in all of their divisions, for like, ever.
gmc was marketed as a heavier duty vehicle than the chevy. In fact they still use the slogan- "professional grade".
I do remember driving a 70-ish c-30 longhorn (extended long bed) into a bay during my gas station job days, and being astounded that it had coil springs in the rear with a crazy long and heavy control arm.
chevrolet
pontiac
oldsmobile
buick
cadillac
Was the suggested "trade up" order for gm shoppers.
When they devised that, lasalle would have been between buick and cadillac.
This explains why chey and pontiac share some things the others don't and that holds true up the line.