The Fairmont from 1978-1979 has very compact units. The evaporator in the car is narrower than the glovebox. It's mounted under the glovebox, on the passenger side. I don't know where the condensor mounts could attach to the Mustang, but the unit is only 1" thick, so it's easy to fit. It's quite wide, too.
This evaporator unit also contains the heater core and fan. It's been a while since I looked at a Mustang floor, but it seems that there is more space in the Mustang than the Fairmont. At worst case, I can see having to punch a couple of holes for the hoses, but I would replace the Mustang heater at the same time.