When I started working on my 68 coupe, I could jack the right front torque box and just lift the right front wheel. Now with a full cage, weld in sub frame connectors and a bolt in x brace, the same jack point lifts the whole right side of the car. Stiff as a board. The connectors aren't through the floor, either.
Do a google search on "Mustang roll cage" and click the Images tab. you will find a lot of builds that include connectors both under and through the floor.
Some Mustang Forums:
http://www.allfordmustangs.com/forums/
http://www.moddedmustangs.com/forums/
Dale