Back on a PC and can type.
So you are sure the center cable is MIA right? The center cable on the pre-87 cars differs from the 87+ Cars. The handle in the 87+ had a self-adjuster built into the handle. On the early cars the actual center cable had an adjustment nut.
I haven't worked on mamy pre-87 cars so it excapes me what the cable looks like. The M-2810-A center cable might work, but it doesn't come with a tensioner equalizer bar. You would need to source one, and i'm not sure if the 87+ one would work. Nobody makes the pre-87 center cable though, so not an easy task. You may need to make your own hybrid center cable out of the adjustable M2810 cable (8mm nut threads onto this BTW) and the equalizer bar from an 87+
The other two cables linked at LMR will connect to that bar and go to the disk brakes. They route along the "football" guides under the car that were the same on 79-92 mustangs.
One other peice you might want to get
Easily replace your broken or missing 1994-2004 Mustang parking brake cable guides with this parking brake cable guide kit!
lmr.com
If you head a junkyard these are on every 79-04. They bolt to the sway bar mount on the control arm and pull the parking brake cable away from the rear tires to prevent rub.