What do you know about it? There is a good chance that it doesn’t exist anymore however. The fact that it is a '66 Mustang ups the odds that it has found good homes throughout the years but most cars get crushed and recycled at some point.
The car you describe was apparently a daily driver and well worn by the time your father sold it. Chances are slim that the buyer poured money into it for restoration. It probably went to the junkyard and got parted out if lucky. What you need is a surrogate. Find something as nearly identical in equipment and paint it the same color. It could still be a family heirloom that you maintain and pass on.