1975 Omega need help please

Hello, I need your help. I have a 75 Omega and it is nearly October, I live in the mountains of Colorado and have not heater in my car. I bought the heater core and have a friend who will put it in. The problem is the first attempt, he could not find how to access the core without messing with the airconditioner. Any suggestions? Please? Anyone ever done this on this make and year before? Please help if you can, it's going to be a c-c-c-co-o-o-llllld-d-d-d winter for me if I can't get this fixed. Thanx