I think it has more to do with they were putting different engines in the GMC's back then. You could get a big block V-6 in a GMC. These engines were all about torque and pulling power. From what I can see and read they did out last the straight six's of the chevies, but for good reason. And, they had plenty of draw backs of there own. I'm sure they did't get the fuel milage the straight six's got and they would be more trouble to shift and drive because of the low rpm power. I would rather have a 230 than a 305 V-6.
Just me opinion though.