I've had the K code book for many years now. Good book, but much is out of date, so I am always verifying against other sources.
The small block book by Bob Mannel has some interesting information in appendix D. He states that K code Mustangs came standard with radio resistance spark plug wires, and that solid core wires were available as a regular production option. Also includes a table showing part numbers, lettering colors, and years of use.
-Scott