The cruise control setup for 68 is in Osbornes Assembly manuals....I'm sure it's similar. I have heard there are slight differences between the 67 and 68, not sure what they are though. My 68 has factory cruise, it's made by Perfect Circle. Do you have cruise in your car?
If you get hold of a copy of the original vacuum/wiring diagrams it shows how to set up the factory unit. There were two different systems, one dealer and one factory installed for 67. The 67 is harder to come by since items like the dash switch, under column wire tray and turn signal lever are 67 year only items. If you need more help send me a private e-mail and I can get you in touch with the guy who's literally writing the book on Mustang cruise.