The 65 will fit on the 66, although the filler panel will cover the very bottom of the speedometer where there are no numbers. As for the wiring I THINK they are the same, but I'm not sure as 1965 had idiot lights and 1966 had actual dials. You can find the wiring diagrams online. From what I could find it will work, it's just that you will have to change some colours around. The clocks wire the exact same way in both years, but the tach is different. In 1965, the tach connects to a red with green stripe wire, and a pink wire. Here is a picture of the 1966 schematics.
Here are 2 links. The first shows how the 1966 rally pac works. The second is a demo of a colourized 1965 wiring manual. Page 7 shows the wiring for the 1965 rally pac.
http://midlife66.com/wiring/66acces1.jpg http://www.fordmanuals.com/estore/demo/Demo10015.pdf
So you can use it, your just going to have to change where the tachometer connects.