Is a GMC truck just a dressed up Chevy truck?

I owned (well, my mom and me) two of them. A 70 4 barrel in a lesabre and a 72 2 barrel in a skylark.

The v6 in those mid 60's gmc trucks was 304 cubic inches IIRC. Looked like a V8 except for the spark plug and exhaust port count.
The early GMC truck V6s were from 305 all the way to 478 cubes.