You may want to go slow. If you can cut out and replace one piece at a time trying to keep the car structurely sound as you go. If you start hacking away at a uni-body it will start to flex as the supporting structure is cut. If you are going to have it mounted so you can spin it, don't forget to install door braces, or leave the doors on and closed as you cut the floor out to help keep the shape. You are taking on a big project, but it can be done right with a little skill and a lot of pre-planning. One place that helped me out a lot was... http://together.net/~manycj/mustang/mustang1.htm
Check this out, he did a total rip down and build up and had the smarts to photograph every step. If a picture is worth a thousand words, this guy's site could fill a couple books. My 69 is down to the bare shell and I'm about to do the boxes' and floor so I feel your pain bro. You won't have to rip out so much hair if you GO SLOW.