I did one years ago on my '64 Marauder. At the time you could get all the parts from Ford.
http://www.cjponyparts.com/TechControlValve.asp
There is a how to from CJ pony ( actually a Mustang Monthly article)
It isn't hard per se, just disassemble. CLEAN REAL GOOD!. replace worn parts usually the bronze insert in the tube and seals gaskets etc reassemble and adjust. The biggest issue I had was that area is really grundgy and the small hoses can be a bitch to get off. They most likely should be replaced any way if they are old. Good Luck Stu