I just recently changed mine out in my 76 GMC, and I only had 3 nuts on studs coming through the firewall. When looking from the front, one of them is on the top right of the box that has your evaporator core in it, one is on the bottom just to the left of the one on the top, and the last one is kind of on the left, and as mentioned above, it's a pain in the butt to get to. I couldn't get to it with anything I had without dropping the inner fender. Fortunately that is not a difficult task. I just removed the tire and then droped the inner fender enough to get to the bolt. Then, if memory serves correctly, there was one bolt going through from the inside. I did wonder if there were other bolts because it was fairly difficult to get the ductwork out, but it was just being stubborn. I just had to yank a little harder. I think it took me somewhere in the neighborhood of 5 hours to change out my heater core.
BTW, I don't know if you have one or not, but my Haynes Repair Manual was not a real big help on this particular project. I don't know if Chilton's would be better or not.
Hope this helps...
Mike
