I'm not too familiar with the older generations (although I hope one day I'm as lucky as you are to come across such a classic
). Perhaps this will give you a stepping stone on your project.
I found a .pdf for the '67 with stock AC and replacing the heating core. That can be seen HERE
Note that this will take you directly to the .pdf file.
I got that "how-to" from THIS
Apparently to replace the heater core is not as easy task. It doesn't look un-doable and seems like you won't need to remove the dash (just the glove box). The "hard part" seems that it is just a tight place to work and the installation of the new heater core takes some patience.
I also found THIS
which is a write up on installing factory grade AC in a 1967-'68 Mustang.
Book wise, take a browse amongst this site: Ford Books by Model, Engine or Shop Manual Year
They should have a few different manuals that will help, including the shop manual, engine restoration and I'm sure there is a book devoted to interior work. I also like to recommend Haynes over Chilton as they tend to have a more general break down of the car and from what I heard they go a bit more in depth. Although both Chilton and Haynes are short on information it seems. Shop manuals really help.