aside from the engine, GMC's had a range of different features that were "heavier duty" than the Chevs, and always have appealed to those looking for a "truckier" truck - I think the dealers then were the same as now, split between the different divisions, w/ certain car makes going w/ each truck make, and I'm sure folks choosing one or the other was just as irrational as it is today - whatever way the family always leaned!
Bill
