I've seen where trucks have been in crashes, like a rollover, and a new cab was installed. The body shop should have made the paper work right, but sometimes they don't. I am not claiming that your truck went through this process, but things happen during the lifetime of a truck that make finding out the details a little troublesome. I don't personally see that the GMC "actual" manufacturing entity makes your truck worth any less than it being the "prestigious" Chevy vintage. I think that the only major difference besides trim between the two is that one has 4 headlights, and the other has 2. I worked at the Van Nuys Assembly plant for awhile, and I can tell you for sure that I couldn't tell an El Camino from a GMC Sprint, coming down the assembly line. Of course, I was working in an area before the body went on.
It is only the badges, and some different colors, in the end.