Attached are a couple of pictures out of the 1964 Color and Trim Book.
According to these pages (provided by Veronica Sczbecki) the interior trim selections were limited to:
42,45,46,49
56
82, 85, 86,89
However, the Mustang Production Guide also has interior trim codes listed for:
22,25,26
32, 35, 36
Can anyone confirm that these latter codes were also available for 1964.5?