I really doubt that a dealer would switch the engines. I was working in my dad's Chevrolet dealership in those days and we didn't have much trouble getting the engines that we wanted and, even if we didn't have what we wanted, we could and did trade with other nearby dealers that did.
Because of the different VIN, it would have been questionable to register the vehicle with something other that what the factory installed and it would have been impossible to submit a warranty claim for a V8 engine if the VIN showed it should have had a 6. It's not impossible, but I think it is not very probable that a dealer was involved.
As to the color, I think that all the engines were grey, but I can't give a reference to support me beyond an old man's memory.
I guess that I would suggest that the engine switch was done later in the life of the truck.
Steve
