If I remember correctly, there is only one ebrake cable.  Each end attaches to a wheel and the ebrake handle is in the middle.  Have you tried to adjust the cable?  The cable adjustment nut is under the handle.  You need to pop open the cover and you should see it.  I would think as long as the cable is not siezed, it still is good.  Also your rear shoes may need some adjusting as well.  I would try both those adjustments before replacing the cable.  Good luck

Parking brake cables do stretch, and sometimes the individual strands of the cable will rust and gradually break one at a time, so that fewer strands remain to carry the load, so continued use stretches the remaining ones. That's without considering wear to any parts that the cable slides over or pivot holes enlarging, the cable ends gradually pulling out of their ferrules, etc.
 
I checked and the early Pintos only had one cable that was looped from wheel to wheel with the adjuster pulling on the center of the loop. 

I don't understand why the mechanic told you that tightening the cable further would adversely effect the rear shoes' position, as tightening a too-loose cable further would only make the shoes contact the drums when the cable was pulleed taut anyway.

I seem to recall that I had to do something with one of my Pintos in a similar situation, and that the adjustment nut was already bottomed on the end of the threaded portion of the adjustment rod, as yours seems to be. The cable was ok, just stretched.  I used a spacer of some sort directly UNDER the adjustment nut to pull the rod farther out so that the nut could still be used to adjust the rod, and it was still adjustable and working for many years afterwards, even after I got rid of the car. It's been enough years now that I don't remember what I used, and it may just have been a stack of several washers.