I realize this was posted just under a month ago, but to answer your question, "no", in reference to needing drum spindles for the 13" Cobra rotors.
The recently released components that makes it possible to put Cobra rotors and twin-piston alumium PBR calipers on '68-'73 Mustang Disc Brake Spindles or '75-'80 Granada/Monarch Disc Brake Spindles are these conversion components:
http://img35.imageshack.us/img35/7610/cobragranadakit.jpg
http://img22.imageshack.us/img22/6484/granadacobratestcar2.jpg
steve@ultrastang.com