There has been several posts on this swap lately. I have posted quite a bit about my own experience switching from manual drums to power disc/ drums including pictures of the firewall and, in my case, 4 speed brake pedal. The Automatic would be the same change. Rather than me repost and repeat what I did, PM me and I can help that way. Again, this is from my own experience, so not necessarily the only way.
Also there have been post regarding the proportioning valve that could be required to be switched. Again there are alternatives like using a separate adjustable rear brake valve with the original distribution block you now have, or switching to a new combo valve mentioned in the posts.
Geoff.
EDIT:
OK, David was typing at the same time I was. He has very good info there in his post. However you may NOT need to replace your front spindles depending on which brand of brakes you go with. Several kits are available that utilize the original spindles. I went with SSBC's 4 piston Kelsey Hayes style that work very well with standard 14" wheels. You have some research to do, but a worthwhile upgrade in my humble opinion.