In the foot actuated assembly is a "U" shaped fitting with a round hole and a slot that the cable end with a ball fits into. The cable must be disconnected from the rear brake cables. There is a "U" shaped clip with ears that holds the cable sheath to the housing and a special clip that keeps the ball end in place.
I recomend that you get a 1973 Ford Service Manual set and a copy of the 1973 Chassis and Body Assembly Manuals available most Mustang mail order stores. As those are the only two Assembly Manuals available for 1973, you can use the 1972 Interior and Electrical manuals with about 90 pct confidence of applying to your year.