Mike (52 ashton),
I cannot check this now, but: last Fall, before I packed things away for hibernation, I think I found a Zenith carb for a 270 that fit a 235/261 intake manifold.
My guess for an additional reason why the GMC Zenith carbs do not find their way onto Chevy stovebolt-sixes (and Rochesters find their way onto GMC sixes) is the matter of availability (there are many more car/truck Rochesters to be found) and the matter of cost (a big difference in $$).
