About The 1965 Mustang
The first Mustang rolled off the production lines in March 1964. The first year resulted in some confusion over the model year and it has subsequently been called the 1964, 1964 1/2, and 1965 models. For clarification purposes this article will discuss the later 1965 Mustang, i.e. the Mustang produced in the latter half of the year that had options the first production run of the Mustang early in March did not have. This Mustang is frequently referred to the 1964 ? model. There were changes to the 1964 1/2 model so that is why the model year is frequently broken into halves. It helps to determine what type of 1965 Mustang is being referred to. Some of the changes midyear include the 260 engine being replaced by the 289 cid V8, adjustable driver seats, revised spare tire brackets, an alternator instead of a generator, and other changes as well.
Source: Mustang Forums