Is a GMC truck just a dressed up Chevy truck?

In the early 80s my brother picked up from a coworker a '67 GMC 1 ton (I guess that would make it a 3500) flatbed dually with a 351 V6, 4 speed, and Detroit locker. It got horrible gas mileage but nowadays would be a cool truck to find again. Dad bought it from my brother, then Dad sold it in the late 80s.
Those engines all had a really unique sound.