So if 261 was a factory engine in anything smaller than a C60 would it not have been a well know fact by the old timers?
If you are referring to the 261 ever being an option in smaller that C60 series trucks,
this page from the 1962 GM Chevrolet Truck Specification Manuals shows "yes" the 261 was supposedly available as an option in smaller than 60 series trucks.
For comparison with a few other years:
1963 engine applications (box at bottom of page - no 261 listed)
1961 6-cylinder truck engine applications (box at top of page - 261 only available as standard engine in C60)
