If I was you I would just get a good water pump from any mustang parts dealer. I am not so sure about the cheap ones from local parts stores. You don't ever want to be out on the road and lose your water pump.
It's not the fact that it's going mustang to mustang. What matters is the size and type of the engine that counts. Yes the inlet and outlet should be corresponding with the radiator in/out but you can work around that with aftermarket flex hose. The previous posts are correct though water pumps are inexpensive so the expense of buying hoses will be close to the cost of the right pump. Oh and the '76 is a mustang two body style and the '79 is the first year of the foxbody.