Tom (bluebird) - Zenith made an excellent carburetor (they still do). One reason you hear little good is that they were not used on most light-duty pickups (too expensive!), so most mechanics are simply not that familiar with the carburetors. Also, parts can be a problem locally (no problem with parts using the internet).
Checked the Carter Master manual, and can find no data as to CFM on any of the YF carbs. It simply was not published. However Chevrolet used the YF on the 250 in some trucks in the 1965 to 1967 years. I believe the base will be larger than the base on your manifold.
I would give some thought to a Zenith, either original or a new replacement. It is an excellent unit, and everything fits like it was made to fit. Also, if you ever do anything for increased performance, most of the Zenith units have replaceable venturi, so a larger venturi could be installed. The Carter (or Rochester) units have fixed venturi.
Jon.
