I don't think dwg 21 has the prop cable length on it either, it just specifies where the holes are in the firewall and has a note that says "DETERMINE EXACT LOCATIONS FOR YOUR OWN ENGINE INSTALLATION USING COAT HANGER WIRE TO MEASURE THE FIREWALL TO TERMINATION DISTANCE."
If you register your S/N on the webstore, you should be able to access all the current drawings for your airplane online, rather than having to buy the usb disk. I can send you dwg 21 if you can't access it there.