a lot of info here
I'll check all that.
About the GM generic name, it is a facility for us because GM somewhat exists as make, but note that for many other cases we do not use a generic name but we rather pick a model randomly. The most common being Dodge Diplomat and its clone.
Note that we do that even for some GM : the Chevrolet Express vs GMC Savana for example, even if not identifiable properly we do not list these as GM.
We did that only when we had a common name (C/K-Series, Suburban).
As Chevrolet seems much more common than GMC for these older trucks, I would list these as Chevrolet unless they can be identified as GMC.