If you want "correct", you'll have to find a 66 Mustang C4. The 65 C4 had internal differences, and of course in 67 the shift pattern changed. If you just want it to fit and work, most C4 from 65 well in the 70's will "fit". If the trans in question was not configured for floor shift, you'll have to install the floor shift selector arm, which will add about $75 to your cost.