Is a GMC truck just a dressed up Chevy truck?

Most famously the olds 403 was used in "6.6" badged trans am's circa 1977-81.

Trans am's with pontiac engines had "400" badging.

I'm fairly certain that the 305 didn't show up until the next body style- 1982, and by then you couldn't get any pontiac engine in that car, but you could get a "corporate" chevy 2.8 or even a 151 4 cyl.

Just learned the 151 was a pontiac design.