For the number of '64½ Mustang with the F-code 260 V8, use the Mustang Production Guide by Jim Smart and Jim Haskell. They do a statistical analysis of all the cars in their database, with a percentage of the number of U-code 170, F-code 260, D-code 289, and K-code 289 cars. Multiply that percentage (I think the 260 V8 is around 40% or so) by the total number of known '64½ production (121,538 cars), and you'll have a decent estimate. Remember, it's not 100% accurate since they have a non-random sampling of the initial Mustang population. Their sample is only those they have found or had reported to them, so it's not random. But I would guess this method would get you within about 5-10% of the true value.
As for '65's with a 260, I know that one: 0.
'64½ Indy 500 Pace Car coupe</font color=blue> /forums/images/icons/cool.gif