I've done a little research in this area, but could never really nail it down fully. I was working on a tilt version, but frankly I don't see any reason why it would be any different than a non-tilt short shaft with rag joint system.
For this I found that C7OZ 3C577-B was used for the Mustang. A copy of area from the 1967 MPC is attached. Yes, it does say "tilt steering wheel". What is interesting is that there is no insulator listed for any other Mustang application. This is hard to believe considering the harsh environment in that area, and the fact that the steering column tube would direct that towards the driver's face.
The only place that I found the part in reproduction form is here:
http://www.mustangsetc.com/67-mustang-steering-column-insulator-w-tilt/For completeness sake, regarding the long shaft no rag joint version, there is a seal of sorts from what I've seen. I added a picture from the shop manual that shows the seal. I can't find any part numbers though. All of the shop manual detailed pictures with part numbers are of the rag joint version. I suspect that someone else can chime in here and provide a part number for this. I also suspect that this would be a carry over part from 1966.
What is more interesting is the shop manual figure 7 (also attached), which shows the 3C577 insulator on a short shaft non-tilt column, which adds some credence to my thought that the C7OZ 3C577-B would be used on all of the short shaft columns, whether tilt or not.