I was looking at a 64.5 mustang that had an F code Vin but had a 289 engine (also had the 289 logo on teh front quater panels). The owner said it was correct as the 260 was ready yet or something like that when the car was made. It sounded like a bunch of BS to me. Is there any other reason why a 289 would be in an F code car other than someone along the way wanted a better engine in the car. In other words, Ford didn't do this for any reason did they?

f = 260

I'm pretty sure if it were a 289 it would have been C (2v) or D (4v) in a 64 1/2
this reverted to C, A, or K for 65 - 67.

funny story. check on the datestam of the engineblock, if that says 66 then you know it's BS. But i can't verify that story and I don't really believe it either
the datestamp is when you stand in front of the engine just to the front of the driver's side head (between ditributor and oilpressure sensor).
it's stamped on a little flange. first digit designates the year