Firstly..
Why not use one of the 50 bagillion manual racks that are out there that actually fit the car? A 79-80 mustang HAD a manual rack.
Secondly, why not use one of the 25 bagillion aftermarket manual racks that are out there that actually fit the car?
Lastly, a MII rack uses a different mount point/structure than what is on a fox k member...even if you have a free one, the trouble to make it work won't be worth it.