Todays What is it question

It took some "thinking" "what haven't I seen?"

I knew it was not Mopar because of the dist drive / gear setup
I wasn't sure about some sixes
At first thought maybe it was something like the experimental Olds or Ponch hemis

And then it just came to me --a guess---"what about" that damned old GMC?
Well I knew it would stump about 99% percent of everyone here. But I figured with all the knowledge here, someone would bail it. Congratulations.