I have not done one but own a converted car and have replaced harnesses. The engine and pcm harness are separate from the body harness I think. All you should need is a5.0 computer and engine harness. It's pretty easy. The dash harness is not changed.
Also I know on the older fox's they had a switch on the gauge cluster near the right hand side at the top that you can switch it so your rpm gauge would be accurate, Don't know if its like that on the newer ones, the one I had was a 79.