I replaced the full floor in my 70 Charger a few yrs ago and also added the USCartool frame connectors. I did not add any bracing to the car when I cut out the floor. The engine, trans, and rear end were out of the car. I leveled the car up on about 6 or 8 jacks stands and wood blocks. I left the doors on the car so I could check to make sure the car wasn't flexing by opening and closing the doors. My frame rails were solid, but if yours are rusty at all, I'd weld in some temporary braces. I added the frame connectors after the floor was in, and it did require some minor grinding to get it to fit flush to the floor. If that's the connectors you are using, I would definitely install after the new floor pan.

What I did for the e-brake cable was mark the routing of the cable then mocked up the connector to the floor. I figured out the angle and height that the cable would need to go straight through the connector. I then ran a piece of tubing through the frame connector at an angle, welded it in and ground it smooth. It came out pretty well I think. Unfortunately the car still isn't finished though...

Rich