If you have an 87-89 this is the book for you. The 90-93 is very similar but there is wiring differences. I have this manual and it is the real ford service manual. It is approximately 3000 pages of every piece of the car.
What is the size of the restrictor? I know I don't have one in mine and I just replaced the heater core I don't want to bust it due to too much pressure
See http://home.netcom.com/~skent/mustang/1990/heater_core/hcore.html for more information on the restrictor and how to make one if you don't already have one.