I have never done one on a 68 so I can only speculate. I will say that the biggest challenge I can imagine will be to get it around any existing heater box installed. But if you remove as much as possible (glove box, etc.) you may be able to squeeze it over the heater box and then with a screw driver snug it up behind the heater box.
Also, many of the replacement firewall pad/insulators don't always line up with the holes, and the clips tend to not work so well and be difficult to push through. My guess is there is some sweat in your foreseeable future but patience and perseverance will be your friend.