Sorry, I lost track of this thread, didn't see your reply.
1) when I did this, I chose to cut the dash for the center vent and the balls on the sides, looked like it was easier than ripping the dashboard out. Cutting the holes for the balls was a PITA, though, it was a show quality car and I was deathly afraid of boogering it up.
2) I recommend you acquire a donor car, as complete as possible. It's going to be pretty much ruined, so this is a possibly big expense.
3) Get a Polaroid camera and plan on buying lots of film. Take a lot of pictures of the donor car as you strip it.
4) When you get the donor car stripped, you'll cut out a big piece of the firewall and use it for a template, some of the bolt holes are common between the two and these will locate the template as you patch/cut your firewall.
5) plan on replacing most of the functional parts you take off the donor, unless it's got Freon in it when you get it you can count on the expansion valve being stuck, the hoses leaky, etc.
6) I've worked on cars all my life and this was the biggest and hardest job I ever did. I will say that it would be a lot easier if the car isn't already restored, most of the ones I did were already finished and working on them at this level was difficult due to the danger of even a little scratch getting on the car. On the first one I was grinding on the firewall when I realized the sparks were going straight up in the air and landing on the roof, pitting the new multi-thousand dollar black paint....