I have a friend who builds full-tube chassis and roll cages and pretty much anything high-performance. Anyway, he happens to have a 65 Fastback in his shop right now that be built a full-tube chassis for, but that differs from your situation since you're keeping the original chassis but adding a roll cage.
As far as building a roll cage, I wouldn't suggest you doing one unless you know how to weld well and are good with metal fabrication. You're going to want to weld to the frame though, not just the floor. So like you'd wanna put your new floors in, and then mark on the floors where the frame runs under it. Because the idea of a roll cage is to protect you if your car goes into a roll, and if you're just welding it to your floor sheet metal, it'll tear that up in a heartbeat with a 3000 pound car coming down on it. I plan to build a roll cage for my 67 Coupe sometime, but my next project is an engine hoist and engine stand all in one, and it will be able to fold up for easy storage to boot!
Anyway, places like Mustangs Plus sell premade roll cages. I'm not sure how good they are so I can't recommend them and I can't put them down either.
Lastly, MAKE SURE YOUR FRAME RAILS ARE GOOD!! If your rails are no good, then the finest roll cage in the world will do you no good, because it will just go right through the rails on impact. A good idea is to tear out any panels you know need replaced, then take the car to be sandblasted. The sandblasting will show you any other panels that may need to be replaced that you might not have saw earlier. Metal is a very tricky thing, and what looks strong is not always strong.
Any other questions, go ahead and shoot em by me.